Add prometheus end-point

[proffalken]

Many of us use Prometheus to monitor the rest of our infrastructure, it would be great to include a /metrics endpoint so we can scrape the various results for each site monitored

I'm a python guy, not a Node developer, and I'm struggling to understand how Vue hangs together, but in essence you'd want to add https://github.com/siimon/prom-client to the project and then setup custom metrics

I'd recommend converting the Friendly Name to lower-case and replacing any special characters with underscores, then add that as a custom label so we can either pull just the individual metrics out or sum the whole lot very easily.

If this does get added, then #84 could add the remote executor name as a custom label as well, providing the ability to split the metrics based on target name, remote executor, or anything else that gets added in future.