Collect metrics for 'git nav-downstream'

Check that everything works with git nav-downstream, which we're not afraid of breaking

Bug: 832386
Change-Id: I7c7561c749bc72cc12804c4ebce8f827ed8be02b
Reviewed-on: https://chromium-review.googlesource.com/1134518Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

gclient.py

@@ -2845,8 +2845,7 @@ def CMDverify(parser, args):
 
 
 @subcommand.epilog("""For more information on what metrics are we collecting and
-why, please read metrics.README.md or visit
-<short link to metrics.README.md in gitiles>.""")
+why, please read metrics.README.md or visit https://bit.ly/2ufRS4p""")
 def CMDmetrics(parser, args):
   """Reports, and optionally modifies, the status of metric collection."""
   parser.add_option('--opt-in', action='store_true', dest='enable_metrics',

git_nav_downstream.py

@@ -13,8 +13,10 @@ import argparse
 import sys
 
 from git_common import current_branch, branches, upstream, run, hash_one
+import metrics
 
 
+@metrics.collector.collect_metrics('git nav-downstream')
 def main(args):
   parser = argparse.ArgumentParser()
   parser.add_argument('--pick',