Add basic metric yml defintion:

  - Add examples for metric definition
  - Use json schema validator
  - Use method_missing for defining methods for attributes

config/metrics/counts_28d/deployments.yml

+name: deployments
+description: Total deployments count for recent 28 days
+value_type: integer
+stage: release
+status: data_available
+default_generation: generation_1
+full_path:
+  generation_1: counts_monthy.deployments
+milestone: 13.2
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/35493
+group: 'group::ops release'
+time_frame: 28d
+data_source: database
+distribution: [ee, ce]
+tier: ['free', 'starter', 'premium', 'ultimate', 'bronze', 'silver', 'gold']

config/metrics/counts_7d/g_project_management_issue_title_changed_weekly.yml

+name: g_project_management_issue_title_changed_weekly
+description: Distinct users count that changed issue title in a group for last recent week
+value_type: integer
+product_category: issue_tracking
+stage: plan
+status: data_available
+default_generation: generation_1
+full_path:
+  generation_1: redis_hll_counters.issues_edit.g_project_management_issue_title_changed_weekly
+milestone: 13.6
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/issues/229918
+group: 'group::project management'
+time_frame: 7d
+data_source: redis_hll
+distribution: [ee, ce]
+tier: ['free', 'starter', 'premium', 'ultimate', 'bronze', 'silver', 'gold']

config/metrics/counts_all/deployments.yml

+name: deployments
+description: Total deployments count
+value_type: integer
+stage: release
+status: data_available
+default_generation: generation_1
+full_path:
+  generation_1: counts.deployments
+milestone: 8.12
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/735
+group: 'group::ops release'
+time_frame: all
+data_source: database
+distribution: [ee, ce]
+tier: ['free', 'starter', 'premium', 'ultimate', 'bronze', 'silver', 'gold']

config/metrics/license/recorded_at.yml

+name: recorded_at
+description: When the Usage Ping computation was started
+value_type: string
+product_category: collection
+stage: growth
+status: data_available
+default_generation: generation_1
+full_path:
+  generation_1: recorded_at
+milestone: 8.10
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/557
+group: group::product analytics
+time_frame: none
+data_source: ruby
+distribution: [ee, ce]
+tier: ['free', 'starter', 'premium', 'ultimate', 'bronze', 'silver', 'gold']

config/metrics/license/uuid.yml

+name: uuid
+description: GitLab instance unique identifier
+value_type: string
+product_category: collection
+stage: growth
+status: data_available
+default_generation: generation_1
+full_path:
+  generation_1: uuid
+  generation_2: license.uuid
+milestone: 9.1
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/1521
+group: group::product analytics
+time_frame: none
+data_source: database
+distribution: [ee, ce]
+tier: ['free', 'starter', 'premium', 'ultimate', 'bronze', 'silver', 'gold']

config/metrics/schema.json

+{
+  "type": "object",
+  "required": ["name", "description", "value_type", "status", "default_generation", "full_path", "group", "time_frame", "data_source", "distribution", "tier"],
+  "properties": {
+    "name": {
+      "type": "string"
+    },
+    "description": {
+      "type": "string"
+    },
+    "value_type": {
+      "type": "string",
+      "enum": ["integer", "string", "number", "boolean"]
+    },
+    "product_category": {
+      "type": ["string", "null"]
+    },
+    "stage": {
+      "type": ["string", "null"]
+    },
+    "status": {
+      "type": ["string"],
+      "enum": ["data_available", "planned", "in_progress", "implmented"]
+    },
+    "default_generation": {
+      "type": "string"
+    },
+    "full_path": {
+      "type": "object"
+    },
+    "milestone": {
+      "type": ["number", "null"]
+    },
+    "milestone_removed": {
+      "type": ["number", "null"]
+    },
+    "introduced_by_url": {
+      "type": ["string", "null"]
+    },
+    "group": {
+      "type": "string"
+    },
+    "time_frame": {
+      "type": "string",
+      "enum": ["7d", "28d", "all", "none"]
+    },
+    "data_source": {
+      "type": "string",
+      "enum": ["database", "redis", "redis_hll", "prometheus", "ruby"]
+    },
+    "distribution": {
+      "type": "array",
+      "items": {
+        "type": "string",
+        "enum": ["ee", "ce"]
+      }
+    },
+    "tier": {
+      "type": "array",
+      "items": {
+        "type": "string",
+        "enum": ["free", "starter", "premium", "ultimate", "bronze", "silver", "gold"]
+      }
+    }
+  }
+}

config/metrics/settings/database_adapter.yml

+name: adapter
+description: This metric only returns a value of PostgreSQL in supported versions of GitLab. It could be removed from the usage ping. Historically MySQL was also supported.
+value_type: string
+product_category: collection
+stage: growth
+status: data_available
+default_generation: generation_1
+full_path:
+  generation_1: database.adapter
+group: group::enablement distribution
+time_frame: none
+data_source: database
+distribution: [ee, ce]
+tier: ['free', 'starter', 'premium', 'ultimate', 'bronze', 'silver', 'gold']

lib/gitlab/usage/metric.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Usage
+    class Metric
+      include ActiveModel::Model
+
+      InvalidMetricError = Class.new(RuntimeError)
+
+      attr_accessor :default_generation_path, :value
+
+      validates :default_generation_path, presence: true
+
+      def definition
+        self.class.definitions[default_generation_path]
+      end
+
+      def unflatten_default_path
+        unflatten(default_generation_path.split('.'), value)
+      end
+
+      class << self
+        def definitions
+          @definitions ||= Gitlab::Usage::MetricDefinition.definitions
+        end
+
+        def dictionary
+          definitions.map { |key, definition| definition.to_dictionary }
+        end
+      end
+
+      private
+
+      def unflatten(keys, value)
+        loop do
+          value = { keys.pop.to_sym => value }
+          break if keys.blank?
+        end
+        value
+      end
+    end
+  end
+end

lib/gitlab/usage/metric_definition.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Usage
+    class MetricDefinition
+      METRIC_SCHEMA_PATH = Rails.root.join('config', 'metrics', 'schema.json')
+
+      attr_reader :path
+      attr_reader :attributes
+
+      def initialize(path, opts = {})
+        @path = path
+        @attributes = opts
+      end
+
+      # The key is defined by default_generation and full_path
+      def key
+        full_path[default_generation.to_sym]
+      end
+
+      def to_h
+        attributes
+      end
+
+      def validate!
+        self.class.schemer.validate(attributes.stringify_keys).map do |error|
+          Gitlab::ErrorTracking.track_and_raise_for_dev_exception(Metric::InvalidMetricError.new("#{error["details"] || error['data_pointer']} for `#{path}`"))
+        end
+      end
+
+      alias_method :to_dictionary, :to_h
+
+      class << self
+        def paths
+          @paths ||= [Rails.root.join('config', 'metrics', '**', '*.yml')]
+        end
+
+        def definitions
+          @definitions ||= load_all!
+        end
+
+        def schemer
+          @schemer ||= ::JSONSchemer.schema(Pathname.new(METRIC_SCHEMA_PATH))
+        end
+
+        private
+
+        def load_all!
+          paths.each_with_object({}) do |glob_path, definitions|
+            load_all_from_path!(definitions, glob_path)
+          end
+        end
+
+        def load_from_file(path)
+          definition = File.read(path)
+          definition = YAML.safe_load(definition)
+          definition.deep_symbolize_keys!
+
+          self.new(path, definition).tap(&:validate!)
+        rescue => e
+          Gitlab::ErrorTracking.track_and_raise_for_dev_exception(Metric::InvalidMetricError.new(e.message))
+        end
+
+        def load_all_from_path!(definitions, glob_path)
+          Dir.glob(glob_path).each do |path|
+            definition = load_from_file(path)
+
+            if previous = definitions[definition.key]
+              Gitlab::ErrorTracking.track_and_raise_for_dev_exception(Metric::InvalidMetricError.new("Metric '#{definition.key}' is already defined in '#{previous.path}'"))
+            end
+
+            definitions[definition.key] = definition
+          end
+        end
+      end
+
+      private
+
+      def method_missing(method, *args)
+        attributes[method] || super
+      end
+    end
+  end
+end

spec/lib/gitlab/usage/metric_definition_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Usage::MetricDefinition do
+  let(:attributes) do
+    {
+      name: 'uuid',
+      description: 'GitLab instance unique identifier',
+      value_type: 'string',
+      product_category: 'collection',
+      stage: 'growth',
+      status: 'data_available',
+      default_generation: 'generation_1',
+      full_path: {
+        generation_1: 'uuid',
+        generation_2: 'license.uuid'
+      },
+      group: 'group::product analytics',
+      time_frame: 'none',
+      data_source: 'database',
+      distribution: %w(ee ce),
+      tier: %w(free starter premium ultimate bronze silver gold)
+    }
+  end
+
+  let(:path) { File.join('metrics', 'uuid.yml') }
+  let(:definition) { described_class.new(path, attributes) }
+  let(:yaml_content) { attributes.deep_stringify_keys.to_yaml }
+
+  it 'has all definitons valid' do
+    expect { described_class.definitions }.not_to raise_error(Gitlab::Usage::Metric::InvalidMetricError)
+  end
+
+  describe '#key' do
+    subject { definition.key }
+
+    it 'returns a symbol from name' do
+      is_expected.to eq('uuid')
+    end
+  end
+
+  describe '#validate' do
+    using RSpec::Parameterized::TableSyntax
+
+    where(:attribute, :value) do
+      :name               | nil
+      :description        | nil
+      :value_type         | nil
+      :value_type         | 'test'
+      :status             | nil
+      :default_generation | nil
+      :group              | nil
+      :time_frame         | nil
+      :time_frame         | '29d'
+      :data_source        | 'other'
+      :data_source        | nil
+      :distribution       | nil
+      :distribution       | 'test'
+      :tier               | %w(test ee)
+    end
+
+    with_them do
+      before do
+        attributes[attribute] = value
+      end
+
+      it 'raise exception' do
+        expect(Gitlab::ErrorTracking).to receive(:track_and_raise_for_dev_exception).at_least(:once).with(instance_of(Gitlab::Usage::Metric::InvalidMetricError))
+
+        described_class.new(path, attributes).validate!
+      end
+    end
+  end
+
+  describe '.load_all!' do
+    let(:metric1) { Dir.mktmpdir('metric1') }
+    let(:metric2) { Dir.mktmpdir('metric2') }
+    let(:definitions) { {} }
+
+    before do
+      allow(described_class).to receive(:paths).and_return(
+        [
+          File.join(metric1, '**', '*.yml'),
+          File.join(metric2, '**', '*.yml')
+        ]
+      )
+    end
+
+    subject { described_class.send(:load_all!) }
+
+    it 'has empty list when there are no definition files' do
+      is_expected.to be_empty
+    end
+
+    it 'has one metric when there is one file' do
+      write_metric(metric1, path, yaml_content)
+
+      is_expected.to be_one
+    end
+
+    it 'when the same meric is defined multiple times raises exception' do
+      write_metric(metric1, path, yaml_content)
+      write_metric(metric2, path, yaml_content)
+
+      expect(Gitlab::ErrorTracking).to receive(:track_and_raise_for_dev_exception).with(instance_of(Gitlab::Usage::Metric::InvalidMetricError))
+
+      subject
+    end
+
+    after do
+      FileUtils.rm_rf(metric1)
+      FileUtils.rm_rf(metric2)
+    end
+
+    def write_metric(metric, path, content)
+      path = File.join(metric, path)
+      dir = File.dirname(path)
+      FileUtils.mkdir_p(dir)
+      File.write(path, content)
+    end
+  end
+end

spec/lib/gitlab/usage/metric_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Usage::Metric do
+  describe '#definition' do
+    it 'returns generation_1 metric definiton' do
+      expect(described_class.new(default_generation_path: 'uuid').definition).to be_an(Gitlab::Usage::MetricDefinition)
+    end
+  end
+
+  describe '#unflatten_default_path' do
+    using RSpec::Parameterized::TableSyntax
+
+    where(:default_generation_path, :value, :expected_hash) do
+      'uuid'                                     | nil    | { uuid: nil }
+      'uuid'                                     | '1111' | { uuid: '1111' }
+      'counts.issues'                            | nil    | { counts: { issues: nil } }
+      'counts.issues'                            | 100    | { counts: { issues: 100 } }
+      'usage_activity_by_stage.verify.ci_builds' | 100    | { usage_activity_by_stage: { verify: { ci_builds: 100 } } }
+    end
+
+    with_them do
+      subject { described_class.new(default_generation_path: default_generation_path, value: value).unflatten_default_path }
+
+      it { is_expected.to eq(expected_hash) }
+    end
+  end
+end