Add an enabled FF for safety

An improperly configured histogram can create problems with Prometheus.
In order to make triage easier if there is a problem, this commit
introduces the `ci_pipeline_creation_step_duration_tracking` feature
flag.

config/feature_flags/development/ci_pipeline_creation_step_duration_tracking.yml

+---
+name: ci_pipeline_creation_step_duration_tracking
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/68485
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/339486
+milestone: '14.2'
+type: development
+group: group::pipeline execution
+default_enabled: true

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

@@ -88,8 +88,10 @@ module Gitlab
           end
 
           def observe_step_duration(step_class, duration)
-            metrics.pipeline_creation_step_duration_histogram
-              .observe({ step: step_class.name }, duration.seconds)
+            if Feature.enabled?(:ci_pipeline_creation_step_duration_tracking, default_enabled: :yaml)
+              metrics.pipeline_creation_step_duration_histogram
+                .observe({ step: step_class.name }, duration.seconds)
+            end
           end
 
           def observe_creation_duration(duration)

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

@@ -343,19 +343,38 @@ RSpec.describe Gitlab::Ci::Pipeline::Chain::Command do
   end
 
   describe '#observe_step_duration' do
-    it 'adds the duration to the step duration histogram' do
-      histogram = double(:histogram)
-      duration = 1.hour
-
-      expect(::Gitlab::Ci::Pipeline::Metrics).to receive(:pipeline_creation_step_duration_histogram)
-        .and_return(histogram)
-      expect(histogram).to receive(:observe)
-        .with({ step: 'Gitlab::Ci::Pipeline::Chain::Build' }, duration.seconds)
-
-      described_class.new.observe_step_duration(
-        Gitlab::Ci::Pipeline::Chain::Build,
-        duration
-      )
+    context 'when ci_pipeline_creation_step_duration_tracking is enabled' do
+      it 'adds the duration to the step duration histogram' do
+        histogram = double(:histogram)
+        duration = 1.hour
+
+        expect(::Gitlab::Ci::Pipeline::Metrics).to receive(:pipeline_creation_step_duration_histogram)
+          .and_return(histogram)
+        expect(histogram).to receive(:observe)
+          .with({ step: 'Gitlab::Ci::Pipeline::Chain::Build' }, duration.seconds)
+
+        described_class.new.observe_step_duration(
+          Gitlab::Ci::Pipeline::Chain::Build,
+          duration
+        )
+      end
+    end
+
+    context 'when ci_pipeline_creation_step_duration_tracking is disabled' do
+      before do
+        stub_feature_flags(ci_pipeline_creation_step_duration_tracking: false)
+      end
+
+      it 'does nothing' do
+        duration = 1.hour
+
+        expect(::Gitlab::Ci::Pipeline::Metrics).not_to receive(:pipeline_creation_step_duration_histogram)
+
+        described_class.new.observe_step_duration(
+          Gitlab::Ci::Pipeline::Chain::Build,
+          duration
+        )
+      end
     end
   end
 end