Add pipeline size by pipeline size histogram metric

This commit adds a new metric that we can use to measure a pipeline
total size (the amount of builds in a pipeline) grouped by a pipeline
type (like `push` or `schedule` etc).

doc/administration/monitoring/prometheus/gitlab_metrics.md

@@ -43,6 +43,7 @@ The following metrics are available:
 | `gitlab_cache_operation_duration_seconds`                      | Histogram |                   10.2 | Cache access time                                                                                   |                                                     |
 | `gitlab_cache_operations_total`                                | Counter   |                   12.2 | Cache operations by controller/action                                                               | `controller`, `action`, `operation`                 |
 | `gitlab_ci_pipeline_creation_duration_seconds`                 | Histogram |                   13.0 | Time in seconds it takes to create a CI/CD pipeline                                                 |                                                     |
+| `gitlab_ci_pipeline_size_builds`                               | Histogram |                   13.1 | Total number of builds within a pipeline grouped by a pipeline source                               |                                                     |
 | `job_waiter_started_total`                                     | Counter   |                   12.9 | Number of batches of jobs started where a web request is waiting for the jobs to complete           | `worker`                                            |
 | `job_waiter_timeouts_total`                                    | Counter   |                   12.9 | Number of batches of jobs that timed out where a web request is waiting for the jobs to complete    | `worker`                                            |
 | `gitlab_database_transaction_seconds`                          | Histogram |                   12.1 | Time spent in database transactions, in seconds                                                     |                                                     |

lib/gitlab/ci/pipeline/chain/command.rb

@@ -77,19 +77,18 @@ module Gitlab
             bridge&.parent_pipeline
           end
 
-          def duration_histogram
-            strong_memoize(:duration_histogram) do
-              name = :gitlab_ci_pipeline_creation_duration_seconds
-              comment = 'Pipeline creation duration'
-              labels = {}
-              buckets = [0.01, 0.05, 0.1, 0.5, 1.0, 2.0, 5.0, 20.0, 50.0, 240.0]
-
-              Gitlab::Metrics.histogram(name, comment, labels, buckets)
-            end
+          def metrics
+            @metrics ||= Chain::Metrics.new
           end
 
           def observe_creation_duration(duration)
-            duration_histogram.observe({}, duration.seconds)
+            metrics.pipeline_creation_duration_histogram
+              .observe({}, duration.seconds)
+          end
+
+          def observe_pipeline_size(pipeline)
+            metrics.pipeline_size_histogram
+              .observe({ source: pipeline.source.to_s }, pipeline.total_size)
           end
         end
       end

lib/gitlab/ci/pipeline/chain/metrics.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Ci
+    module Pipeline
+      module Chain
+        class Metrics
+          include Gitlab::Utils::StrongMemoize
+
+          def pipeline_creation_duration_histogram
+            strong_memoize(:pipeline_creation_duration_histogram) do
+              name = :gitlab_ci_pipeline_creation_duration_seconds
+              comment = 'Pipeline creation duration'
+              labels = {}
+              buckets = [0.01, 0.05, 0.1, 0.5, 1.0, 2.0, 5.0, 20.0, 50.0, 240.0]
+
+              ::Gitlab::Metrics.histogram(name, comment, labels, buckets)
+            end
+          end
+
+          def pipeline_size_histogram
+            strong_memoize(:pipeline_size_histogram) do
+              name = :gitlab_ci_pipeline_size_builds
+              comment = 'Pipeline size'
+              labels = { source: nil }
+              buckets = [0, 1, 5, 10, 20, 50, 100, 200, 500, 1000]
+
+              ::Gitlab::Metrics.histogram(name, comment, labels, buckets)
+            end
+          end
+        end
+      end
+    end
+  end
+end

lib/gitlab/ci/pipeline/chain/sequence.rb

@@ -27,6 +27,7 @@ module Gitlab
               yield @pipeline, self if block_given?
 
               @command.observe_creation_duration(Time.now - @start)
+              @command.observe_pipeline_size(@pipeline)
             end
           end
 

spec/lib/gitlab/ci/pipeline/chain/sequence_spec.rb

@@ -56,11 +56,24 @@ describe Gitlab::Ci::Pipeline::Chain::Sequence do
     end
 
     it 'adds sequence duration to duration histogram' do
-      allow(command).to receive(:duration_histogram).and_return(histogram)
+      allow(command.metrics)
+        .to receive(:pipeline_creation_duration_histogram)
+        .and_return(histogram)
 
       subject.build!
 
       expect(histogram).to have_received(:observe)
     end
+
+    it 'records pipeline size by pipeline source in a histogram' do
+      allow(command.metrics)
+        .to receive(:pipeline_size_histogram)
+        .and_return(histogram)
+
+      subject.build!
+
+      expect(histogram).to have_received(:observe)
+        .with({ source: 'push' }, 0)
+    end
   end
 end