OpenTelemetry Metrics: Emit an UpDownCounter (gauge) for active handlers

[mauroservienti]

As proposed in #6953 (comment)

[bbrandt]

Example of active handlers from one of our apps:

Total Active Handlers

sum (nservicebus_messaging_active_handlers{kubernetes_namespace=~"$namespace", app_kubernetes_io_name=~"$service"})

Active Handlers Per Service

sum by(app_kubernetes_io_name) (nservicebus_messaging_active_handlers{kubernetes_namespace=~"$namespace", app_kubernetes_io_name=~"$service"})