Add usage data counter for network policy packet stats

This MR adds redis based usage data counter that will be used for
collecting usage metrics for network policy packet stats

ee/lib/gitlab/usage_data_counters/network_policy_counter.rb

+# frozen_string_literal: true
+
+module Gitlab::UsageDataCounters
+  class NetworkPolicyCounter < BaseCounter
+    KNOWN_EVENTS = %w[forwards drops].freeze
+    PREFIX = 'network_policy'
+
+    class << self
+      def add(forwards, drops)
+        return unless Gitlab::CurrentSettings.usage_ping_enabled?
+
+        Gitlab::Redis::SharedState.with do |redis|
+          redis.multi do
+            redis.incrby(redis_key(:forwards), forwards)
+            redis.incrby(redis_key(:drops), drops)
+          end
+        end
+      end
+    end
+  end
+end

ee/spec/lib/gitlab/usage_data_counters/network_policy_counter_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Gitlab::UsageDataCounters::NetworkPolicyCounter, :clean_gitlab_redis_shared_state do
+  describe '.add' do
+    it 'increases drops and forwards counters' do
+      described_class.add(10, 5)
+      expect(described_class.totals).to eq(network_policy_forwards: 10, network_policy_drops: 5)
+
+      described_class.add(2, 1)
+      expect(described_class.totals).to eq(network_policy_forwards: 12, network_policy_drops: 6)
+    end
+  end
+end