Add metrics for `max_concurrent_connections` enforcement

Followup to  #3281 / #3285
 
- A counter for times the semaphore couldn't be acquired immediately (i.e. [`try_acquire`](https://docs.rs/tokio/1.48.0/tokio/sync/struct.Semaphore.html#method.try_acquire) before async `acquire`)
- Gauges for [`available_permits`](https://docs.rs/tokio/1.48.0/tokio/sync/struct.Semaphore.html#method.available_permits): absoluate and % of max.