Collectors can now be async

[Half-Shot]

Fixes #139

[jaller94]

Does refresh need a lock (or wrapper returning an ongoing promise) so it cannot get run more than once at a time?

[Half-Shot]

I think we're okay in this case. The caller should manage that, whether that be a bridge or the application requesting the /metrics endpoint. I don't think a race will cause any serious problems as you're just asking it to refresh metrics , which should be RO functions.

[jaller94]

Back and forth in my head:
You seem to be pretty sure that all of these promises are fulfilled within seconds.
I'm a bit worried that a bridge calls long-running (or never-ending) functions on registration or refreshing.
Then again, that would be a (performance) bug in the bridge..
Making this async is still be an improvement, because other things can run in between.

[Half-Shot]

Then again, that would be a (performance) bug in the bridge..
Making this async is still be an improvement, because other things can run in between.

This was my thinking. A bridge should ensure that collector functions do not block for serious amounts of time, and if they do then the /metrics endpoint should show that clearly by taking longer rather than us trying to make the endpoint return faster but with potentially the wrong values.

[jaller94]

Looks good to me. 👍