Add support for dumping in OpenMetrics format

[danobi]

As promised, here is support for dumping in OpenMetrics format. I did my best to structure the commits so it's more easily reviewed, but here are some salient points:

• I opted not to use a 3rd party library for generating openmetrics b/c it's really quite trivial to do ourselves. It also frees us from any constraints the library puts on us. There are quite a few contraints from the popular ones b/c they probably didn't envision this use case. They're mostly designed for applications to report metrics about themselves.
• It's much easier to review this PR if you go commit-by-commit. There aren't actually that many changes. Most of the diff is the model registration in the final ~6 commits.

[facebook-github-bot]

@brianc118 has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[dschatzberg]

All my comments are of the pretty minor variety. This all looks great. Once the inline-comments are addressed we can import on our end

[danobi]

I've addressed the review changes in fixup commits so it's easier to see the delta. Just lemme know if you want me to git rebase --autosquash it.

[dschatzberg]

Thanks, this all looks good to me - not need to squash, we'll import on our end

[facebook-github-bot]

@brianc118 has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[facebook-github-bot]

@brianc118 merged this pull request in facebookincubator/below@895770a8136e21cadd7847f9fb7c615069cd79b8.