[autotest] Collect time used for call devserver_healthy

BUG=None
TEST=local run

Change-Id: If4c93d342fc40f9c1040b17a2fec44e8fc527940
Reviewed-on: https://chromium-review.googlesource.com/353402
Trybot-Ready: Dan Shi <dshi@google.com>
Tested-by: Dan Shi <dshi@google.com>
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
diff --git a/client/common_lib/cros/dev_server.py b/client/common_lib/cros/dev_server.py
index 1b4f0de..4c69c4c 100644
--- a/client/common_lib/cros/dev_server.py
+++ b/client/common_lib/cros/dev_server.py
@@ -75,6 +75,8 @@
 
 DEFAULT_SUBNET_MASKBIT = 19
 
+_timer = autotest_stats.Timer('devserver')
+
 class MarkupStripper(HTMLParser.HTMLParser):
     """HTML parser that strips HTML tags, coded characters like &amp;
 
@@ -333,6 +335,7 @@
 
 
     @classmethod
+    @_timer.decorate
     def devserver_healthy(cls, devserver,
                           timeout_min=DEVSERVER_SSH_TIMEOUT_MINS):
         """Returns True if the |devserver| is healthy to stage build.