Cache .WithLabelValues results?

[JustinAzoff]

Instead of

          mReceived.WithLabelValues(intf, workerString).Set(..
          mDropped.WithLabelValues(intf, workerString).Set(..
          mIfDropped.WithLabelValues(intf, workerString).Set(..

cache the result of WithLabelValues at the start of the worker.