Addition of Prometheus Metrics to Enhance Observability

[xmcqueen]

It is essential to get real run-time metrics to properly integrate kubernetes-event-exporter into a large scale architecture. At a minimum the following provide enough data to get a strong signal for service health:

1. if the event exporter is up
2. How many events are being received
3. How many errors are coming from the watcher
4. How many errors are coming from the sender

Having these metrics allows us to deploy this on a large scale and have automation determine if there are any problems.

I've got a PR in testing now that exposes these with the standard Prometheus tooling.

Please let me know if there are any problems or concerns, or if you are a user of kubernetes-event-exporter and are interested in seeing this functionality.