The sharding team is adding support for multiple databases and in order
for us to have good visibility into performance we'll want to be able to
distinguish between the different databases in our prometheus metrics.

Adding a prometheus label allows us to filter/group by the different
databases.

By default there will only be 1 `db_config_name` which is `primary` by
default. But this is configured by `config/database.yml` and in future
there will be 2 databases called `main` and `ci`.

This MR also ensures the new label is disabled by default to allow us to
introduce this to production in a slightly more controlled way. We opted
to use an environment variable instead of feature flag in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/66885 because it's
too expensive to query the databases so frequently to check the feature
flag value.