autotest: fix syntax error in host_scheduler exception handler

BUG=chromium:719778
TEST=None

Change-Id: I2f0c58d776152a849356d6b70c39a9219777c6ef
Reviewed-on: https://chromium-review.googlesource.com/499491
Commit-Ready: Aviv Keshet <akeshet@chromium.org>
Tested-by: Aviv Keshet <akeshet@chromium.org>
Reviewed-by: Shuqian Zhao <shuqianz@chromium.org>
diff --git a/scheduler/host_scheduler.py b/scheduler/host_scheduler.py
index 1980da6..b9e5739 100755
--- a/scheduler/host_scheduler.py
+++ b/scheduler/host_scheduler.py
@@ -501,7 +501,7 @@
         # for host-scheduler role. Thus do not send email for it.
         raise
     except Exception:
-        metrics('%s/uncaught_exception' % _METRICS_PREFIX).increment()
+        metrics.Counter('%s/uncaught_exception' % _METRICS_PREFIX).increment()
         raise
     finally:
         email_manager.manager.send_queued_emails()