Add Prometheus metrics for using_database_replica #98

As we're adding a fairly critical database utility in #95, we'd like to be able to effectively monitor its operation with Prometheus.

Implementation

Add the following counter:

• flagsmith_db_selected_replica_count_total, labeled with result (success/failure) and replica_name.

Inside using_database_replica:

• On each successful ensure_connection() call, increment the counter: flagsmith_db_selected_replica_count_total.labels(result="success", replica_name= chosen_replica).inc()
• In each OperationalError exception handling branch, increment the counter: flagsmith_db_selected_replica_count_total.labels(result="failure", replica_name=attempted_replica).inc()

Check how existing metrics are used and defined by searching for an existing metric, e.g. flagsmith_task_processor_enqueued_tasks_total.

Refer to developer documentation to learn how to develop and test Prometheus metrics.