Request to have metrics emitted on health check failures

### What should we add or change to make your life better?

Requesting metrics in some of the default classes such as:
https://github.com/microsoft/reverse-proxy/blob/04b1fe8d8cd0f25b814805c2fb91c68fb26c7f29/src/ReverseProxy/Health/DestinationHealthUpdater.cs#L43

https://github.com/microsoft/reverse-proxy/blob/04b1fe8d8cd0f25b814805c2fb91c68fb26c7f29/src/ReverseProxy/Health/ActiveHealthCheckMonitor.cs#L197C31-L197C31

https://learn.microsoft.com/en-us/dotnet/core/diagnostics/metrics-collection

Specifically, today, I'm looking for a way to have alerts go off when our health checks fail without reimplementing YARP's default implementations. More generally it would be nice if YARP had metrics to complement the Log statements.

### Why is this important to you?

We are able to set up alerts on metrics but not on individual log messages. Metrics can alert the on-call engineers to look at the logs for further information.