Telemetry [:commanded, :application, :dispatch]

[davydog187]

Adds telemetry to the dispatcher, which will add start and stop events
to all dispatches.

• Documentation
• Tests

Contributes to #387

[slashdotdash]

It would be possible to support [:commanded, :application, :dispatch, :exception] events if the aggregate returned the stacktrace on exception. This is currently being ignored by the ExecutionContext.format_reply/3 function so the stack trace is not returned to the dispatcher process.

Do you think it is worth adding the :exception telemetry event so that it supports all of the telemetry span events (:start, :stop, :exception)?

[davydog187]

I do think it would be worth adding the exception event