Sample database metrics for replica hosts

What does this MR do and why?

In !75793 (merged), we updated the database host metrics to include all connection classes, defined in Gitlab::Database.base_database_models, not just ActiveRecord::Base.

As a follow up to that, this MR extends the metrics collection to include statistics for replica hosts if any are configured.

Related to #346909 (closed)

How to set up and validate locally

1. You can collect the metrics for your configured database hosts by executing the following in the rails console:

   sampler = Gitlab::Metrics::Samplers::DatabaseSampler.new
   sampler.sample

   The method will return an array containing a hash of the labels and stats for each host.
2. You can then view and inspect the Prometheus metrics by running:

   sampler.metrics

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.