Measure detailed MR status service duration

What does this MR do and why?

This MR adds performance monitoring to the DetailedMergeStatusService by measuring and logging the execution duration of key operations. This helps identify performance bottlenecks in merge request status checks.

Changes made:

• Added measure_duration method to wrap operations with timing measurements
• Instrumented verify_all_mr_mergeability_checks and check_ci_status operations
• Added feature flag log_detailed_merge_status_duration_enabled to control logging
• Logs include operation name, duration, merge request ID, and project name

References

• Related work item: #585222

Screenshots or screen recordings

Not applicable - this is a backend performance monitoring change with no UI impact.

How to set up and validate locally

1. Enable the feature flag in the Rails console:

 Feature.enable(:log_detailed_merge_status_duration_enabled)

2. Call the list MR endpoint (http://gdk.test:3000/api/v4/groups/:id/merge_requests/), and tail the logs like so tail -f log/application_json.log | jq you should see a log message like so

{
  "severity": "INFO",
  "time": "2026-02-23T23:29:18.830Z",
  "correlation_id": "01KJ6DBX627BJXEN6NSYCD5F63",
  "meta.caller_id": "GET /api/:version/groups/:id/merge_requests",
  "meta.remote_ip": "172.16.123.1",
  "meta.feature_category": "code_review_workflow",
  "meta.http_router_rule_action": "classify",
  "meta.http_router_rule_type": "SESSION_PREFIX",
  "meta.root_namespace": "gpt",
  "meta.client_id": "ip/172.16.123.1",
  "event": "merge_requests_detailed_merge_status_service_executed",
  "operation": "check_ci_status",
  "duration_s": 0.00015500000154133886,
  "merge_request_id": 3657,
  "project": "gitlabhq1"
}