Add Telemetry Tracking for Users Associating Milestones to Releases

What does this MR do and why?

Adds metric for the number of unique users associating releases to milestones.

Also adds index for `(author_id, id, created_at) to speed the query.

Related to #235798 (closed)

Database migrations and queries

Please see in comments to relevant lines.

MR acceptance checklist

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