Add open connections PollingCounter

[benaadams]

There are counters for requests per second, total requests, failed requests and current requests; however there isn't a counter for the number of open connections (e.g. including connected clients in a keep-alive, but currently idle state)

This would be a good value to report.

/cc @davidfowl @NickCraver

[analogrelay]

#4769 seems to cover this, right? We're planning to look at that in the 5.0 timeline (as part of diagnostics for YARP). If that issue doesn't cover it well, we can expand it.

[NickCraver]

There are built-in counters for the others but indeed I'd like to see connections. We'd definitely add them to our default metrics set for all Kestrel-enabled apps. I'm personally confused by #4769 because it's presented as events instead of counters and I'm not sure how that related to the current Kestrel counters available.

[analogrelay]

it's presented as events instead of counters

For all our counters we also emit an event (when the EventSource is enabled). There are definitely counters listed there too.

I'm not sure how that related to the current Kestrel counters available.

That would be @davidfowl implementing those counters without checking the existing plan ;). We can certainly update the issue though.

[benaadams]

I'm not sure how that related to the current Kestrel counters available.

That would be @davidfowl implementing those counters without checking the existing plan ;). We can certainly update the issue though.

The hosting counters (requests/s, requests in progress etc) work for all servers (i.e. Kestrel, IIS, HttpSys); however Hosting can't provide the current connection count because that works differently for all servers

[analogrelay]

Right. The issue I linked is for Kestrel specifically.

[BrennanConroy]

Should be fixed by #21649