Feature request: Add prometheus formatted output

[Z3hka]

This is a followup on #140
For me (and probably every other sysadmin out there running matrix and using prometheus for monitoring) it would be helpful if the federation tester would add a third output format in the prometheus syntax (at least as an opt in) so it can be used to monitor the health of prometheus from a blackbox perspective. I understand you don't want automatic requests on the hosted federation tester but I (and probably other too) would be happy to self-host a federation tester instance if that allows me to get this health data into prometheus.
I hope for this to be doable as it does not add any real complexity to the federation tester (it already supports two different syntaxes) but would add a quite a lot of value to it.
Unfortunately I have no experience writing go so i can't just present a pull request with it already implemented.
I hope you consider this addition and let me know if such a PR would be appreciated or you would even consider implementing it yourselfs.
Thanks in advance