Use the guava's EventBus to expose telemetry data from factstore

[bresciamattia]

Is your feature request related to a problem? Please describe.
It would be nice to be able to observe details within the factstore, in order to analyze and log.

Describe the solution you'd like
Being able to listen to the event bus from the server implementation for this telemetry data.
E.G.: receive events from the bus whenever a subscription query starts/ends, with each subscription request.

This should be documented, to allow users to subscribe to those internal events.