Add prometheus metrics

[aawdls]

Add a prometheus metrics endpoint that will enable monitoring of state.

Metrics for:

• Number of graphql subscriptions
• Number of unique clients (if possible?)
• Number of CA and PVA monitors
• Count of exceptions / errors - of different types if appropriate
• Number of dropped updates

This will enable better monitoring.

[AlexanderWells-diamond]

This has been merged as part of #53