Emit a metric tracking the number of empty responses from prometheus

[drolando-stripe]

We'd like to have a way to monitor the number of Keda errors due to empty responses from prometheus. We've enabled the ignoreNullValues flag for most of our prometheus triggers and we need a way to monitor the number of errors related to this.

Right now this error gets logged but the error metric that Keda emits is generic and doesn't differentiate by error type.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically closed due to inactivity.

[aliaqel-stripe]

Hey folks can we reopen this issue, too?