Measure how long it takes operations using egress connections to write to network

We sometimes see operations get bottlenecked at the point in their lifecycle where they need to perform RPC. In SERVER-64964, SERVER-64965, and SERVER-63261, we're adding metrics to measure how long connection establishment takes, how many operations fail while waiting to acquire connections, and how long operations spend waiting to acquire connections.

For completeness, let's also measure the amount of time operations spend doing networking after they acquire a connection. Add a measurement of how much wall-time passes between an operation acquiring a connection and it completes writing to the wire.