Investigate the cause of long running, high number of UserMergeToRef method calls to Gitaly

This is a continuation of exploring the causes of this incident to find corrective actions: https://gitlab.com/gitlab-com/gl-infra/production/-/issues/5193

Some individual Gitaly host apdex incidents have been linked to this phenomena where large numbers of these method calls are present. Working backwards, the next steps are probably to better understand what user driven traffic is causing these method calls and if this is bad behavior or a systemic issue.