write monotonic stamp as the last element in the stamps list by convention

[ethanrublee]

farm-ng-core/py/farm_ng/core/events_file_writer.py

Line 128 in e67443f

timestamps.append(get_system_clock_now(semantics="log/write"))

As a reader of logs, i'd like to depend on event.stamps[-1] always being the monotonic stamp, as this makes it easy/cheap to sort the events by write order without searching for the stamp by clock name.

[edgarriba]

even probably the semantic value should be different ? like

     timestamps.append(get_system_clock_now(semantics="log/clock/system"))
     timestamps.append(get_monotonic_now(semantics="log/clock/monotonic"))