add constant labels to gauges and cumulative metrics

[paivagustavo]

This PR adds support for constant labels on gauges.

The proposed syntax is:

r := NewRegistry()

 	f, _ := r.AddFloat64Gauge("TestGaugeWithConstLabel",
		WithLabelKeys("k1"), WithConstLabel("const", "same"), WithConstLabel("const2", "same2"))

@rghetia this is my first contribution and as you were the one who created the issue, could you review this?

closes #1112

[rghetia]

@paivagustavo thanks for the PR.

we are making commits to dev branch for now. I'll merge the dev branch to master just before the next release.
So can you please rebase your change to dev?

When you rebase, you will have conflict as I have created LabelKey type which contains key and description. So you will have to change that accordingly.

[paivagustavo]

@rghetia I've rebased to dev and change the target branch (I may have pick some commits from the master that was not on dev yet, is it a problem?)

While doing this, I noticed that the canonicalize was not deterministic and sometimes the TestGauge was failing, I have fixed this and made it more reliable while keeping the same purpose.

[rghetia]

@rghetia I've rebased to dev and change the target branch (I may have pick some commits from the master that was not on dev yet, is it a problem?)

yes, that will be a problem. you can do git rebase --onto dev HEAD~2 since you two commits.

[paivagustavo]

Done, the PR now contains only the correct commits.

[rghetia]

one minor nit. Otherwise LGTM.