Investigate changes in SERVER-64966: Count how many pooled connections are created, but never used

New field added to connPoolStats command output.

Description of Linked Ticket

To learn more about the efficiency of our connection pools and their sizing algorithms, we'd like to know how often we create connections that never end up being used to run operations. Let's add a metric to count the number of connections created by our connection pools that are never used to run an operation; i.e. that expire/are destroyed before they are ever used.