Add client count metrics to coil-egress

[ymmt2005]

What

The number of clients for a coil-egress Pod can be useful metrics for HPA.

How

Add a gauge like this:

coil_egress_clinets{namespace="foo",egress="nat"} 30

Checklist

• [x] Finish implentation of the issue
• [x] Test all functions
• [x] Have enough logs to trace activities
• [x] Notify developers of necessary actions

[ymmt2005]

resolved by #148