As GitLab is planning on moving `ci_*` tables to a separate database we
need to start expanding our observability/logging/metrics to allow us to
distinguish between the different databases being queried.

We had already implemented logging per database by including the
database name (ie. what Postgres calls the database) in the log
statements in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/63490 but later we
learnt this wasn't a good idea because we may want to use 2 different
Postgres servers with the same Database name (in fact that's what our
current migration plan involves). As such we need to use the name that
the GitLab application uses to distinguish between the 2 different
databases. This is the name used to configure the `config/database.yml`
and must be unique.

In the default case it does not have a name and Rails will call it
`primary`. This has the unfortunate result that this MR will add JSON
log keys `db_replica_primary_duration_s` and
`db_primary_primary_duration_s` because we already use the words
`primary/replica` to distinguish between our read-write primary and
read-only replicas. We plan to eventually force the
`config/database.yml` to name these `main` and `ci` and at that time
these log statements will be a little clearer. In the meantime this is
behind a feature flag so should not bother people that aren't intending
to try and make sense of these metrics.