Change message channel back pressure gauge to counter.

[tobias-wilfert]

As part of a recent incident investigation it became clear that having a counter to measure the number of messages in a channel might be nicer than the current gauge. The problem with which is that the metric is only updated if a message is taken out of the channel however if that does not happen for some reason the metric stops.

Note: That changing this also entails channing a decent amount of dashboards, monitors and alerts.