Improve ring buffer diagnostics

The only way currently of knowing if the configured number of endpoints is sufficient to cover the incoming requests, is to see if a BackpressureException occurs.  It would be good to give some more transparency on how full the ring buffer is, ideally split up by query/kv etc.  Perhaps in the form of periodic dumping to log, or an API.  Would be good to give an idea of latency too, e.g. what's the oldest query in the ring buffer.