metrics_pdd_hook: allow committer to specify reason if PDD does not need to be updated

e.g. if metrics.proto was merely reformatted,
or only comments were changed.

Bug: 140895523
Test: create test CL that changes metrics.proto
Change-Id: Ia0430d2870097c5d8a4d53c57f853ffb184b41b3
diff --git a/metrics_pdd_hook.py b/metrics_pdd_hook.py
index 093f2cdf..fa85855 100755
--- a/metrics_pdd_hook.py
+++ b/metrics_pdd_hook.py
@@ -38,7 +38,7 @@
 def is_commit_msg_valid(commit_msg):
     for line in commit_msg.splitlines():
         line = line.strip().lower()
-        if line.startswith('updated-pdd') and 'true' in line:
+        if line.startswith('updated-pdd'):
             return True
 
     return False
@@ -65,11 +65,16 @@
         return 0
 
     print('This commit has changed {metrics_file}.'.format(metrics_file=metrics_file))
-    print('Please update the Wifi Metrics Privacy Design Doc (PDD) at go/wifi-metrics-pdd')
+    print('If this change added/changed/removed metrics collected from the device,')
+    print('please update the Wifi Metrics Privacy Design Doc (PDD) at go/wifi-metrics-pdd')
     print('and acknowledge you have done so by adding this line to your commit message:')
     print()
     print('Updated-PDD: TRUE')
     print()
+    print('Otherwise, please explain why the PDD does not need to be updated:')
+    print()
+    print('Updated-PDD: Not applicable - reformatted file')
+    print()
     print('Please reach out to the OWNERS for more information about the Wifi Metrics PDD.')
     return 1