Remove LabelSet

[c24t]

Following open-telemetry/oteps#90.

@jmacd please check my work here, I may have mangled some of your go examples.

[jmacd]

The api-user.md file is so old that I had to stop looking carefully. I'd like to get this in quickly and continue editing the metrics API spec in future PRs. Thanks!

[c24t]

Thanks for the quick reviews, hopefully this change is still worth making even if the whole file will be overhauled soon.

[arminru]

This PR includes a lot of changes where unrelated line breaks were moved to different positions which makes it hard to spot where content was actually altered or just moved to another line. Please try to avoid this to make reviewing easier. Thanks!

We actually settled for soft breaks a while ago (#192 (comment)) but I also usually keep the style already in place in existing documents.

[jmacd]

Re: line-breaks
Many of these are my doing. I didn't realize there was a soft-breaks policy. @c24t should we remove hard line-breaks in a separate PR across the metrics specs (or the whole repo)?

[c24t]

Sorry about the line breaks @arminru, I re-wrapped paragraphs I edited which made for a lot of spurious line diffs.

should we remove hard line-breaks in a separate PR across the metrics specs (or the whole repo)?

I'd be happy to, I'll put up a PR.

[jmacd]

May we merge this ahead of adding soft-breaks everywhere?
I'm feeling like drafting spec changes to help push open-telemetry/oteps#93 and open-telemetry/oteps#96 forward, but I'd like this merged first.