Migrate gateway metrics from gateway-api-state-metrics repo to exporter repo

[R-Lawton]

WHAT:

Add the current gateway existing metrics from the gateway-api-state-metrics repo to gateway-api-metrics-exporter.
Add corresponding tests for each metrics

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

HOW:

Create a new file for the resource creating the relevant metrics using the prometheus go package
Include help strings as before https://prometheus.io/docs/instrumenting/writing_exporters/#help-strings