OpenMetrics unit support (in v1)

[beorn7]

OpenMetrics offers units as part of the metadata. We can easily add them in the various ...Opts next to Help. They would only be used if OpenMetrics is negotiated as the output format (but they will also implicitly show up in protobuf, cf. prometheus/common#214 ).

[vesari]

Hello, I'd like to work on this issue

[beorn7]

/cc @bwplotka @kakkoyun

[bwplotka]

Totally, I am supportive. Help wanted @vesari 😊

[ArthurSens]

Hello, I'd like to work on this issue

Let us know if you need any assistance :)

[vesari]

Great! I'm on it :)

[vesari]

@bwplotka @ArthurSens I've opened my first PR in solving this issue: prometheus/client_model#75. Can you please take a look? I'm also opening PRs in prometheus/common and prometheus/client_golang but they both depend on the former.

[ArthurSens]

Just noticed that Björn was quicker than us with the client_model PR. Feel free to directly ping me in the prometheus/common PR if you need reviewers, I don't receive notifications for that repository 😬