Queue size / in-flight count in metrics listener

[pertu]

It would be nice to have "queued requests count" (aka NioEventLoop.pendingTasks()) in the metrics listener. It's possible to access ApnsClient.channelPool.executor.pendingTasks() using reflection, but it seems too hacky and fragile.

In-flight: easy to calculate using "request start" and "request reject/accept" events, but it would be nice to have it in the default listeners (e.g. MicrometerApnsClientMetricsListener)

[pertu]

In-flight requests can be easily calculated as sent - (accepted + rejected) in monitoring tools (e.g. grafana).

[jchambers]

The metrics listeners are due for an overhaul anyhow; I'll consider this as part of that effort.

[jchambers]

I have, as promised (and after much delay) made a pass at improving metrics listeners in #1046. Having done that, I don't think it necessarily makes sense to add a dedicated metric for outstanding requests because, as you correctly point out, that count

…can be easily calculated as sent - (accepted + rejected) in monitoring tools (e.g. grafana).

Can you please clarify why you'd want to consider "queued requests" seaparately from "in-flight?"

[jchambers]

Closing this issue for now for the reasons stated above and for lack of counter-argument. @pertu if you have the opportunity to provide more information, please feel free to leave a comment here and I'll happily reopen the issue.