Expose Envoy's /stats for statsd agents

[tpaschalis]

This PR closes #7070 connect: Option to Expose Envoy's /stats/ for agents like DataDog that can read from there.

Description of changes

• Add a new StatsBindAddr field to Envoy's BootstrapConfig
• Rename the generatePrometheusConfig to generateMetricsListenerConfig and allow passing in the bind address, the name of the listener, the path that will be exposed and the prefix that will be rewritten
• Add testcases for new functionality
• Run all tests

Comments/Notes

• I initially attempted to configure a single listener, that would expose two routes.
  In that case, we'd have to distinguish between
  • StatsBindAddr == PrometheusBindAddr, where a single listener is needed and
  • StatsBindAddr != PrometheusBindAddr, where two separate listeners should be configured.

This change would have made the code somewhat more complicated, so I thought I'd leave it as two separate listener fields in any case. Let me know what you think!

• A seemingly unrelated test keeps failing.

[banks]

@tpaschalis this PR is awesome, thanks for your effort!

The tests are great and thorough and this is super close.

I had a couple of suggestions inline that mostly revolve around the first point here but just to summarize changes requested:

1. I think we should expose the whole /stats prefix - sorry if that wasn't clear in the issue I probably didn't specify it well enough!
2. Can you add the new config option to the Envoy docs the source is in website/source/docs/connect/proxies/envoy.html.md and you can mostly follow the example set for the prometheus ones.

Thanks again. This is a fantastic PR and much appreciated!

[tpaschalis]

Hey @banks, sorry for pinging you. When you have some time, let me know how this looks ^^

[banks]

Thanks @tpaschalis this looks fantastic!

I had one minor suggestion to fix a comment in the documentation which you can just commit through GitHub UI if that's easiest and then I think this is good to merge!

Oh, we also have our CLA bot complaining that it's waiting for something but I don't see it prompting you to sign the CLA so there might be something stuck there. Paging @alvin-huang is there something we can do to fix CLA bot here?

[tpaschalis]

I've already signed the CLA, and pushed your most recent change directly through the GitHub UI. Let's see how the bot works this time!

[banks]

Looks like CLA bot is happy now! Thanks again. I'll wait for the CI run and then we can get this merged!

[banks]

🎉 Thanks again @tpaschalis, this is a fantastic contribution to Consul!

[banks]

Should end up in Consul 1.7.0 pretty soon!

[tpaschalis]

I'd like to personally thank you. It was my first contribution to Consul, and you helped immensely with your positive attitude and help!! I'm already looking for the next issue to work on!