Remove transaction memory metrics

This removes the `gitlab_transaction_allocated_memory_bytes`
histogram.

This was measuring the memory before and after a transaction. Which
made sense in a single threaded environment, but it doesn't in a
multithreaded one.

It also had high cardinality.

doc/administration/monitoring/prometheus/gitlab_metrics.md

@@ -52,7 +52,6 @@ The following metrics are available:
 | `gitlab_sql_duration_seconds`                                  | Histogram |                   10.2 | SQL execution time, excluding `SCHEMA` operations and `BEGIN` / `COMMIT`                                  |                                                     |
 | `gitlab_ruby_threads_max_expected_threads`                     | Gauge |                       13.3 | Maximum number of threads expected to be running and performing application work                    |
 | `gitlab_ruby_threads_running_threads`                          | Gauge |                       13.3 | Number of running Ruby threads by name                    |
-| `gitlab_transaction_allocated_memory_bytes`                    | Histogram |                   10.2 | Allocated memory for all transactions (`gitlab_transaction_*` metrics)                                |                                                     |
 | `gitlab_transaction_cache_<key>_count_total`                   | Counter   |                   10.2 | Counter for total Rails cache calls (per key)                                                       |                                                     |
 | `gitlab_transaction_cache_<key>_duration_total`                | Counter   |                   10.2 | Counter for total time (seconds) spent in Rails cache calls (per key)                               |                                                     |
 | `gitlab_transaction_cache_count_total`                         | Counter   |                   10.2 | Counter for total Rails cache calls (aggregate)                                                     |                                                     |

lib/gitlab/metrics/transaction.rb

@@ -14,7 +14,6 @@ module Gitlab
       THREAD_KEY = :_gitlab_metrics_transaction
 
       SMALL_BUCKETS = [0.1, 0.25, 0.5, 1.0, 2.5, 5.0].freeze
-      BIG_BUCKETS = [100, 1000, 10000, 100000, 1000000, 10000000].freeze
 
       # The series to store events (e.g. Git pushes) in.
       EVENT_SERIES = 'events'
@@ -43,9 +42,6 @@ module Gitlab
 
         @started_at = nil
         @finished_at = nil
-
-        @memory_before = 0
-        @memory_after = 0
       end
 
       def duration
@@ -56,20 +52,14 @@ module Gitlab
         System.thread_cpu_duration(@thread_cputime_start)
       end
 
-      def allocated_memory
-        @memory_after - @memory_before
-      end
-
       def run
         Thread.current[THREAD_KEY] = self
 
-        @memory_before = System.memory_usage_rss
         @started_at = System.monotonic_time
         @thread_cputime_start = System.thread_cpu_time
 
         yield
       ensure
-        @memory_after = System.memory_usage_rss
         @finished_at = System.monotonic_time
 
         observe(:gitlab_transaction_cputime_seconds, thread_cpu_duration) do
@@ -78,9 +68,6 @@ module Gitlab
         observe(:gitlab_transaction_duration_seconds, duration) do
           buckets SMALL_BUCKETS
         end
-        observe(:gitlab_transaction_allocated_memory_bytes, allocated_memory * 1024.0) do
-          buckets BIG_BUCKETS
-        end
 
         Thread.current[THREAD_KEY] = nil
       end

spec/lib/gitlab/metrics/transaction_spec.rb

@@ -28,14 +28,6 @@ RSpec.describe Gitlab::Metrics::Transaction do
     end
   end
 
-  describe '#allocated_memory' do
-    it 'returns the allocated memory in bytes' do
-      transaction.run { 'a' * 32 }
-
-      expect(transaction.allocated_memory).to be_a_kind_of(Numeric)
-    end
-  end
-
   describe '#run' do
     it 'yields the supplied block' do
       expect { |b| transaction.run(&b) }.to yield_control

spec/lib/gitlab/metrics/web_transaction_spec.rb

@@ -48,16 +48,6 @@ RSpec.describe Gitlab::Metrics::WebTransaction do
     end
   end
 
-  describe '#allocated_memory' do
-    include_context 'transaction observe metrics'
-
-    it 'returns the allocated memory in bytes' do
-      transaction.run { 'a' * 32 }
-
-      expect(transaction.allocated_memory).to be_a_kind_of(Numeric)
-    end
-  end
-
   describe '#run' do
     include_context 'transaction observe metrics'