Publish jmx metrics for connection pool

[shubhamtagra]

Adding following jmx metrics, per host level, should help in debugging leaked connections:

• Total active connections in pool
• Connections borrowed from pool

[harmandeeps]

We can keep discarded connections count too while returning a connection to the pool.

[shubhamtagra]

We can keep discarded connections count too while returning a connection to the pool.

yes, that should also be published.