Add metrics to track load_balancing for Sidekiq workers

Since we allowed Sidekiq workers to use load balancer capabilities we should introduce some metrics.

We should:

1. Introduce the Prometheus metrics that will be used to keep track of how many times we queried primary write log location/replication log location on the request level.
2. Add ::Gitlab::Metrics::Subscribers::ActiveRecord::DB_LOAD_BALANCING_COUNTERS to the structured log
3. Introduces new Prometheus metric in Sidekiq ServerMetrics Middleware that will keep track of executed worker that were using load balancing capabilities, and it would provide information about worker data consistency, which database was chosen (primary or replica), if the job was retried because the replica was not ready, or we have fallen back to the primary.