Change Integer Counter type from AtomicI64 to AtomicU64

[Folyd]

According to the definition of Prometheus's Counter type, the AtomicU64 is preferred to AtomicI64 for IntCounter* types:

A counter is a cumulative metric that represents a single monotonically increasing counter whose value can only increase or be reset to zero on restart.

Fix #364 .

[mxinden]

I am in favor of this change. Thanks.

I find the name IntCounter a bit confusing, as my intuition would include negative numbers as well, which (rightly so) doesn't make any sense for a Prometheus counter. I am failing to come up with a better name.

While already enforced through the type system, I suggest to make the following changes to the type signature comments to emphasize the fact that one can only to pass natural numbers.

[tim-seoss]

It might be useful to also document (not sure where's best, maybe along with the "better performance" notes?), that these integer counters will wrap with inc and inc_by (and that this is acceptable, so long as it doesn't wrap more than once between samples).

Not sure if that's in scope for this PR tho'.

[mxinden]

It might be useful to also document (not sure where's best, maybe along with the "better performance" notes?), that these integer counters will wrap with inc and inc_by (and that this is acceptable, so long as it doesn't wrap more than once between samples).

I am in favor of this, but as you said, I think it is out of scope for this pull request. @tim-seoss would you mind opening up a new pull request doing the suggested changes?

[mxinden]

Unless there are any objections I will merge this pull request later today.

[breezewish]

Thank you!