Provide network metrics for egress connections

The metrics under serverStatus.network only account for the network traffic of ingress connections (see SERVER-6227). Extending the metrics to also include egress connections, those established by a mongo process, would help diagnostics and monitoring. This ticket requires the following:

• Defining new metrics (e.g., egress.physicalBytesOut and egress.physicalBytesIn) for egress traffic.
• Separating egress and ingress metrics in serverStatus.
• Creating a ticket to update the documentation for network metrics in serverStatus.