[client-v2] Metrics for a Connection Pool

[chernser]

There is a need to monitor connections that are created by a client.

Here is list of wanted metrics:
Connection pool (scope)

• connections in use (leased)
• idle connections (available)
• threads waiting for connection (pending)
• total connections (max)

Connection runtime metrics (#2094):

• connection request time (how fast connection is returned from pool, may include creation time)
• connection creation time
• failed connection requests(count, ratio)