Add db_load_balancing_counters to structured_log

What does this MR do?

Adds ::Gitlab::Metrics::Subscribers::ActiveRecord::DB_LOAD_BALANCING_COUNTERS to the Gitlab::InstrumentationHelper.keys.

We introduced counters db_replica_count, db_replica_cached_count, db_primary_count, db_primary_cached_count, but they were not added to the Sidekiq structured log, because it uses Gitlab::InstrumentationHelper.keys

Now, when we introduced load_balancing for Sidekiq, we would like to have this counters in a log as well

Related to: #326225 (closed)

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