feat: metric observer & pool wait timing

[domodwyer]

This PR implements observer-based Pool metric instrumentation, and captures the time taken to acquire a connection permit from the connection pool.

A follow-on from https://github.com/launchbadge/sqlx/pull/1860.

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• feat: metric observer & pool wait timing (b7e797e6)

  This commit adds the public PoolMetricsObserver trait for end users to 
  optionally implement, in order to receive metric / event data from the 
  underlying connection Pool.
  
  PoolMetricsObserver currently exposes a single metric: the duration of time
  spent waiting for a permit from the connection pool semaphore. This allows
  users to quantify the saturation of the connection pool
  (USE-style metrics) and identify when the workload exceeds the available pool
  capacity.
  
  A default no-op method implementation is provided so end-users can choose
  which metrics they care about, and not have to implement the full set of
  methods.
  

[domodwyer]

I think this is good for another pass :+1: