Metrics: Fix timestamps, they were not epoch

Library documentation [0] says this about monotonic():

  The reference point of the returned value is undefined,
  so that only the difference between the results of two calls is valid.

Switching to time().

[0] https://docs.python.org/3/library/time.html

Change-Id: Ia7ef8d89119f7dc72619cac44b213611163ff054
Signed-off-by: Vratko Polak <vrpolak@cisco.com>