Support multiple dbs when monitoring partitions

What does this MR do and why?

Related to #336898 (closed)

In the previous two MRs !68795 (merged) and !70435 (merged) we added multi-database support for partition management (creating new partitions and detaching old partitions) and partition removal (dropping detached partitions).

This MR continues the work by adding multi-database support for partitioning monitoring, by setting up the proper connection before reading metrics from a partitioned model.

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

1. From a rails console, gather the partitioned table metrics:

   Gitlab::Database::Partitioning.report_metrics

2. Also from the rails console, view the partitioned table metrics:

   Gitlab::Metrics.registry.get(:db_partitions_present).values.map { |key, value| [key[:table], value.get] }.to_h

   Should produce something like:

   {"audit_events"=>17, "web_hook_logs"=>10, "incident_management_pending_alert_escalations"=>8, "incident_management_pending_issue_escalations"=>8}

3. From a psql console, you can verify the count of partitions matching the database:

   \d audit_events

   At the bottom of the output, something like:

   Number of partitions: 17 (Use \d+ to list them.)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.