Observe duration of each pipeline creation step

What does this MR do?

This MR adds instrumentation to track and graph the duration in seconds of each pipeline creation step in ::Ci::CreatePipelineService::SEQUENCE. Each class in the sequence is tracked as a separate step.

Issue

#332891 (closed)

Documentation

Details about the new histograms will be added to https://docs.gitlab.com/ee/administration/monitoring/prometheus/gitlab_metrics.html#metrics-available once this MR is merged and we can confirm that the histograms are working as expected.