Add check metrics functionality to promtool

[grobie]

Promtool can check configs and rule files, it'd be great if it also could test the output of a /metrics endpoint and check it for errors to guide developers during the creation of custom exporters.

[beorn7]

Or to guide developers of client libraries… (Ideally, developers of exporters use a client library that protects them from exporting invalid metrics.)

[grobie]

While it existed before #1953, this issue has less information and qualifies as the duplicate.

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.