metrics: Separate metrics collection and print notice logic.

As it is right now, when a function is collecting metrics it prints
the notice and calls sys.exit() to ensure the notice is the last thing
that is printed.

git-cl split has to call git-cl upload multiple times, but once it has
been called once, it exits, so only the first cl is uploaded.

This separates metrics collection from notice printing, so that the
function that is collecting metrics behaves like a function that isn't.
It also makes sure we don't collect metrics for multiple functions at
the same time.

Bug: 868280
Change-Id: Ic58ebe7d19e09ed85fa8b0af76dcbf608ee4c9bc
Reviewed-on: https://chromium-review.googlesource.com/1153503
Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

git_nav_downstream.py

@@ -62,8 +62,5 @@ def main(args):
 
 
 if __name__ == '__main__':
-  try:
+  with metrics.collector.print_notice_and_exit():
     sys.exit(main(sys.argv[1:]))
-  except KeyboardInterrupt:
-    sys.stderr.write('interrupted\n')
-    sys.exit(1)