Add metric to track querying write location on Request level

What does this MR do?

Introduces gitlab_transaction_primary_write_location_count Prometheus counter. This counter will be used to keep track of how many times we queried primary write log location on the request level.

It uses ActiveSupport::Notifications to instrument get_write_location.load_balancing event, and it introduces subscriber Gitlab::Metrics::Subscribers::LoadBalancing that will handle and increment the metrics.

primary_write_location_count will be also added to the lograge and sidekiq structured log.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

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Related to #326225 (closed)