Expose state transition times to prometheus

[PeterJCLaw]

It would be really useful to have the duration which was taken to get a label from one state to the next in Prometheus.
This somewhat be worked out from the history entries post-hoc for analysis, however having them in Prometheus would allow the data to be used for monitoring etc. of both Routemaster itself as well as for whatever is represented by the state machine it is operating.

My current thinking is that each state transition A → B would form a label in a histogram of durations.