Log wal locations counters

What does this MR do and why?

We discussed, if we should remove wal_location_source? One of the reasons is that it does not give us precise information, since we are getting wal location from multiple databases(:ci, :main), and each have it's own primary and replicas. Also, in case when we try to read from the replica, but the replica is not available, we will log wal_location_source: replica, but the actual source would be the primary.

In ActiveRecord::Subscriber we already monitor wal location queries, along with the database name(ci|main) and the database role (replica|primary), so instead of logging:

wal_location_source: primary

we could actually log

          'db_ci_wal_count' => 1,
          'db_ci_replica_wal_count' => 0,
          'db_main_wal_count' => 1,
          'db_main_replica_wal_count' => 0,
          'db_replica_wal_count' => 0,
          'db_primary_wal_count' => 2

This can provide us with enough information, what happened.

Note:

We are already logging all load balancing counters for jobs that are finished (job_status: done) This adds wal_counters also for logs when the job was scheduled (job_status: start)

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