sysmon: Change git/commit_time to a gauge

The git/commit_time metric should not be a String, because we feed it ints.

TEST=Added a unit test. Changing from String -> Gauge makes the test pass.
BUG=chromium:703827

Change-Id: I8300ecee2b37676020495a3c100593d2188a1d68
Reviewed-on: https://chromium-review.googlesource.com/457621
Trybot-Ready: Paul Hobbs <phobbs@google.com>
Reviewed-by: Paul Hobbs <phobbs@google.com>
Tested-by: Paul Hobbs <phobbs@google.com>
diff --git a/scripts/sysmon/git_metrics.py b/scripts/sysmon/git_metrics.py
index 4712700..f3afd6c 100644
--- a/scripts/sysmon/git_metrics.py
+++ b/scripts/sysmon/git_metrics.py
@@ -8,11 +8,11 @@
 from __future__ import print_function
 from __future__ import unicode_literals
 
+import psutil
+
 import os
 import subprocess
 
-import psutil
-
 from chromite.lib import cros_logging as logging
 from infra_libs import ts_mon
 
@@ -20,6 +20,7 @@
 
 
 class _GitRepo(object):
+  """Helper class for running git commands."""
 
   def __init__(self, gitdir):
     self._gitdir = gitdir
@@ -40,12 +41,13 @@
 
 
 class _GitMetricCollector(object):
+  """Class for collecting metrics about a git repository."""
 
   _commit_hash_metric = ts_mon.StringMetric(
       'git/hash',
       description='Current Git commit hash.')
 
-  _commit_time_metric = ts_mon.StringMetric(
+  _commit_time_metric = ts_mon.GaugeMetric(
       'git/commit_time',
       description='Current Git commit time as seconds since Unix Epoch.')