Track duration of each step in one histogram

Tracking them all in separate histograms would make the PE dashboard
very large and slightly messy

lib/gitlab/ci/pipeline/metrics.rb

@@ -14,10 +14,9 @@ def self.pipeline_creation_duration_histogram
         end
         def self.pipeline_creation_step_duration_histogram(step)
-          name = "#{step}_duration_seconds".to_sym
-          description = step.gsub('gitlab_ci_', '').tr('_', ' ')
-          comment = "Duration of the #{description}"
-          labels = {}
+          name = :pipeline_creation_step_duration_seconds
+          comment = 'Duration of each pipeline creation step'
+          labels = { step: step.remove('gitlab_ci_') }
           buckets = [0.01, 0.05, 0.1, 0.5, 1.0, 2.0, 5.0, 10.0, 15.0, 20.0, 50.0, 240.0]
           ::Gitlab::Metrics.histogram(name, comment, labels, buckets)

spec/lib/gitlab/ci/pipeline/metrics_spec.rb

@@ -9,9 +9,9 @@
       expect(::Gitlab::Metrics).to receive(:histogram)
         .with(
-          :gitlab_ci_pipeline_chain_build_duration_seconds,
-          'Duration of the pipeline chain build',
-          {},
+          :pipeline_creation_step_duration_seconds,
+          'Duration of each pipeline creation step',
+          { step: 'pipeline_chain_build' },
           [0.01, 0.05, 0.1, 0.5, 1.0, 2.0, 5.0, 10.0, 15.0, 20.0, 50.0, 240.0]
         )