Use monotonic clock instead of wall time clock for metrics.

From several dashboards, there are high spikes from several
builds where the real time of executing a part of the build
system took many many hours. This is due to using the wall
time clock when calculating the duration. For metrics, the
monotonic clock should be used for more accurate measurement.
Also, cleaned up the time.go file to use the proper proto
functions when marshalling perf protobuf and deleted
unnecessary functions.

Bug: b/143492984
Test: Wrote and ran unit test case.
Change-Id: I1540cfbf37915e3045ecedaa2595c1f9429bcd62