Add timing logging to MergeRequests::RefreshService methods

What does this MR do and why?

This MR adds structured logging for key operations in the MergeRequests::RefreshService to help identify performance bottlenecks. This addresses the performance issues reported in #548046 where UpdateMergeRequestsWorker is not meeting the 10-second SLO targets on GitLab Dedicated instances.

What are the relevant issue numbers?

#548046

Logging Details

The following methods now have their execution time logged to Kibana:

• find_new_commits_duration - Time spent finding new commits
• close_upon_missing_source_branch_ref_duration - Time spent closing MRs with missing source branches
• post_merge_manually_merged_duration - Time spent processing manually merged MRs
• link_forks_lfs_objects_duration - Time spent linking LFS objects from forks
• reload_merge_requests_duration - Time spent reloading merge request diffs
• remove_requested_changes_duration - Time spent removing requested changes (EE only)
• other_method_calls_duration - Time spent on other operations (suggestions, auto-merges, todos, notifications, etc.)

Each method's duration is measured independently using Gitlab::Metrics::System.monotonic_time to provide accurate timing information.

Feature Flag

The logging is controlled by the :log_refresh_service_duration feature flag (experiment type, default: disabled). This allows for safe rollout and testing on specific instances.

To enable:

Feature.enable(:log_refresh_service_duration, User.find_by_username('username'))``