BUG-5280: add executionTimeNanos

BUG-5280: add executionTimeNanos

In order to properly measure impact of requests on the backend
we nead some indication of the complexity involved in servicing
the request. It is not feasible to estimate this by analyzing
the request itself, hence we provide a way for the backend to
communicate how complex it found a request to be back to the
frontend.

Since this measure excludes actor inbox and transport latency,
the frontend can use this measure to weigh the relative complexity
compared to other requests in has sent.

Change-Id: Ia7f435ff8a7fd995a90261b128832c340026de6d
Signed-off-by: Robert Varga <rovarga@cisco.com>