feat(#358): add database connection pool monitoring metrics

[Escelit]

Closes #358

This PR introduces database connection pool monitoring to the MetricsService. Previously, the application lacked visibility into the TypeORM connection pool, making it impossible to detect pool exhaustion under high load.

Changes Made

• Injected the TypeORM DataSource into MetricsService.
• Added three new Prometheus Gauge metrics to track the database pool state in real-time:
  • db_pool_active_connections: The number of clients currently checked out and actively executing queries.
  • db_pool_idle_connections: The number of clients currently idle and available in the pool.
  • db_pool_waiting_queries: The number of queued queries waiting for a database connection to become available.

These metrics utilize prom-client's collect callback functions to dynamically evaluate the state of the Postgres pg.Pool underlying the TypeORM connection, providing efficient and real-time observability.

Type of Change

• New feature (non-breaking change which adds functionality)

Testing

• Verified metrics compile successfully and the collect methods appropriately handle cases where the pool may not be fully initialized or exposed.

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[feyishola]

@Escelit pls resolve conflicts