Concurrently fetch metrics

[AnshulMalik]

Fixes #11

[fogfish]

👍

[fogfish]

@pioneerit can you please also review the proposed changes.

[pioneerit]

As for the PR LGTM.
Perhaps logic with error handling and exit on goroutine via <-ctx.Done() is something I'd double-check. AFAIK the cancel itself won't cause a stop.
At the same time by reading the code, if I understand the idea - the goal is to stop and exit on the first error.

I also looking that our test case didn't cover multiple metrics, so we're not testing concurrency at all.

@fogfish last one, not related to this PR, but just spot a magic number chunkSize. We don't have any comments or docs to it, and the values inside the func, is not the best practice. I'd create small issue and follow up on it.

P.S. I can try to help with test on Wednesday, as today and tomorrow pretty booked already