An interceptor for instrumenting gRPC requests with :telemetry events.

GRPCTelemetry takes one option, the event prefix:

intercept(GRPCTelemetry, event_prefix: [:my, :endpoint])

It will emit two events:

* `[:my, :endpoint, :start]` is emitted the interceptor
is called, it contains the monotonic time in native units
when the event was emitted, called `time`.

* `[:my, :endpoint, :stop]` is emitted after the rest
of the interceptor chain has executed, and will contains
`duration`, the monotonic time difference between the stop
and start event, in native units.

GRPCTelemetry should be added as the first interceptor, so that it instruments the whole request.