Add cluster watcher event metric to monitor the event processing path

[enekofb]

In order to enable users troubleshooting as described in this vision
We want to add metrics to monitor cluster watcher events
So we could monitor explorer write path

AC

• Exists cluster_watcher_event: number of cluster watcher events processed. Useful to determine whether an issue with the event processing infrastructure.
• Exists user documentation in explorer monitoring
• Exists panels in Explorer grafana dashboard (from the documentation)

💡 given this part is implemented via controller-runtime see whether you could leverage already an existing controller-runtime metric with this purpose.