Add db sampler for pg_stat_activity table
What does this MR do and why?
Add db sampler for pg_stat_activity table
This allows the application to periodically snapshot the volume of activity of each endpoints.
We keep track of a list of samples within a 5 minute window to give us flexibility of how far back we wish to observe such samples. This means a maximum of 20 (4 per minute x 5 minutes) samples are stored in the Redis hash. We also store the tally of state per endpoint for flexibility since we may want to drill-down on specific states. e.g. idle-in-transaction for transaction SLI degradations.
feature issue: gitlab-com/gl-infra/scalability#3929
background issues
- gitlab-com/gl-infra/scalability#3818
- https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/3775#note_211133813
- https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/3775#note_2123814996
- https://docs.google.com/document/d/1c4DVX0rDW75bQzo8ngOc8hnGQQyM8Hz1x4z3pNC5wls/edit?tab=t.0#heading=h.5h8ibruat48d
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