Move BBM prometheus metrics into separate class

What does this MR do and why?

Related to #357033 (closed)

Move the prometheus metrics gathering for batched background migrations out of the migration wrapper and into a separate class. This simplifies the migration wrapper a bit, before we begin hoist some of the batching boilerplate into the wrapper.

How to set up and validate locally

1. In a rails console, create an instance of the metrics class, and verify the gauges have no values

   metrics = Gitlab::Database::BackgroundMigration::PrometheusMetrics.new
   metrics.class.metrics[:gauge_batch_size].values # => {}
   metrics.class.metrics[:gauge_sub_batch_size].values # => {}

2. Create a test job and pass it to the metrics instance:

   job = FactoryBot.create(:batched_background_migration_job, started_at: 5.minutes.ago, finished_at: Time.current)
   metrics.track(job)

3. Verify the metrics class has the correct metric values:

   metrics.class.metrics[:gauge_batch_size].values[job.batched_migration.prometheus_labels].get # => 5
   metrics.class.metrics[:gauge_sub_batch_size].values[job.batched_migration.prometheus_labels].get # => 1

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.