Set a Promotheus counter in response to TraceGsmEvent events

These trace event data constructors all indicate when the GSM transitions to a different state.

Karl has asked that the tracer event handling logic in `cardano-node` should react to these transitions by setting a Promotheous counter. Something along the lines of `0` while the GSM is in CaughtUp and `1` (or >0?) when it is not in CaughtUp.