metrics: added prometheus metrics for fleet

[kayrus]

@miekg I've rebased your old PR #1415 and updated godeps

[kayrus]

@joshix could you review docs?

[robszumski]

LGTM, but defer to @joshix

[joshix]

LGTM for purpose.

Defer on merging due to the godeps update. @kayrus and/or fleet team, PTBF

[joshix]

Supersedes #1415

[hectorj2f]

Nice 😉

[kayrus]

@jonboulle this PR is ready to be merged. I don't think tests are necessary for this functionality. At least I didn't find any metrics tests in etcd.

[miekg]

[ Quoting notifications@github.com in "Re: [coreos/fleet] metrics: added p..." ]

@jonboulle this PR is ready to be merged. I don't think tests are necessary for this functionality. At least I didn't find any metrics tests in etcd.

Testing this is pretty annoying, would be nice if prometheus would provide some
kind of metrics recorder that would make this easier...

[jonboulle]

LGTM but let's get v0.12.0 done first?

[tixxdz]

@jonboulle as discussed all v0.12.0 PRs and Issues have been handled!

[kayrus]

Blocks #1537

[tixxdz]

Thanks all, this one was reviewed and already has lgtm! and it touches lot of parts of the code where other PRs may conflict with it. So merging this one and #1537 will be on top later when it's ready and reviewed.