Create new metric for Individual listener status within a Gateway

[R-Lawton]

WHAT:

Create new metrics for Individual listener status within a Gateway. The metric should conform to prometheus guidelines and also align with the pre-existing metrics
Add corresponding tests for each metrics

The metrics should be like-for-like to allow for backwards compatibility.

HOW:

Add the metric to the gateway resource file using the prometheus go package.
Include help strings as before https://prometheus.io/docs/instrumenting/writing_exporters/#help-strings