Add prometheus metrics for batched migrations

What does this MR do?

This introduces a couple of prometheus metrics to track the progress and internal statistics for batched migrations. This will allow us to tune parameters for the migrations better, especially on GitLab.com.

The use case currently is the primary key migrations we're running.

The batched migration feature is behind a feature flag and not yet released. Hence, we might not want to describe the metrics in documentation. I already had that, so there's that for reference:

Data migration metrics

The progress of data migrations can be observed through the following metrics:

Metric	Type	Since	Description	Labels
batched_migration_job_batch_size	Gauge	12.11	Batch size for a batched job	migration_id, migration_identifier
batched_migration_job_sub_batch_size	Gauge	12.11	Sub-batch size for a batched migration job	migration_id, migration_identifier
batched_migration_job_updated_tuples	Counter	12.11	Number of tuples updated by migration job	migration_id, migration_identifier
batched_migration_job_timings	Histogram	12.11	Timings for a batched migration job	migration_id, migration_identifier
batched_migration_job_time_efficiency	Histogram	12.11	Ratio of job duration to interval	migration_id, migration_identifier

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because "migration feature still behind feature flag"
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content