Add OpenMetrics-format endpoint for scraping with Prometheus

[hacklschorsch]

I am creating monitoring infrastructure for the upcoming ​PrivateStorage SaaS. I'd like to add an ​OpenMetrics format statistics endpoint to be scraped by ​Prometheus.

The de-facto industry standard Prometheus scrapes metrics endpoints that by default are available on the things under monitoring on some HTTP server under /metrics.

I see there's already a /statistics?t=json endpoint that seems very similar to what I want to build, and also lots of useful things in misc/operations_helpers/munin. My plan so far is to add a /statistics?t=openmetrics endpoint exporting the same values as the json, but in OpenMetrics?.

In the #tahoe-lafs IRC channel on Libera:

hacklschorsch | Does that make sense?  Is there a better way?
         jpds | hacklschorsch: Yes, a /metrics endpoint would be great
     itamarst | hacklschorsch: that sounds plausible to me

[hacklschorsch]

The code for this is in ​https://github.com/tahoe-lafs/tahoe-lafs/pull/1125 .

[Florian Sesser <florian@…>]

In 6310774/trunk:

Add documentation on OpenMetrics? statistics endpoint.

references ticket:3786

[exarkun]

Merged in ​https://github.com/tahoe-lafs/tahoe-lafs/commit/0a072a98c80f0a9ae1913674b54696ebc992406a