feat: gauge support

[PaulOlteanu]

Description

It'd be nice to have support for Gauge metrics. For example it'd be useful in the relay to see how many actor tasks are running and to notice if actor tasks are being leaked (just an example, not something I necessarily believe).

Breaking Changes

I think none

Notes & open questions

Change checklist

• Self-review.
• Documentation updates following the style guide, if relevant.
• Tests if relevant.
• All breaking changes documented.

[Arqu]

So aside from the nitpick, code looks great, thank you.

Just to give some background why we haven't added these (and maybe it's useful for you too). Gauges are good only if you're interested in a ballpark point in time value to look at live.

In a case where between a single sampling period you have lots of activity but but then also close it all out and end up decreasing to 0, it would look like nothing happened if you plot it as a time series. In reality a lot of stuff happened.

The solution we use and I also generally recommend is running two counters, one for increases and one for decreases. That way you can plot activity on both cases independently, calculate the diff to get the point in time value the same as a gauge, but also be able to see activity on both directions.

Im happy to approve this once the nit is resolved.

[PaulOlteanu]

Yeah using 2 counters is a good point. Usually I like to create some Guard type that wraps a Gauge and increments it on creation and decrements it on drop. I guess that's doable with 2 counters as well.