Enhance observability for exclusive locks

This commit introduces instrumentation for exclusive locks within the
modules `Gitlab::ExclusiveLeaseGuard` and
`Gitlab::ExclusiveLeaseHelpers`, enabling better monitoring and
visibility into lock usage.

When utilizing the `Gitlab::ExclusiveLeaseHelpers#in_lock` method, the
following metrics are now tracked:
- exclusive_lock_count`
- exclusive_lock_wait_duration_s
- exclusive_lock_hold_duration_s

Similarly, when using the
`Gitlab::ExclusiveLeaseHelpers#try_obtain_lease` method, the following
metrics are captured:
- exclusive_lock_count
- exclusive_lock_hold_duration_s

Notably, in the latter method, `exclusive_lock_wait_duration_s` is not
captured as the method returns immediately if the lease is already held,
thus no waiting time is incurred.