sysmon: Fix error reference

BUG=chromium:840873
TEST=Run sysmon for a few ticks

Change-Id: I9882e753abbf4996a97959e26061b857ff6e50d4
Reviewed-on: https://chromium-review.googlesource.com/1050408
Commit-Ready: Allen Li <ayatane@chromium.org>
Tested-by: Allen Li <ayatane@chromium.org>
Reviewed-by: Ningning Xia <nxia@chromium.org>
diff --git a/scripts/sysmon/net_metrics.py b/scripts/sysmon/net_metrics.py
index a5f89a0..e59c8eb 100644
--- a/scripts/sysmon/net_metrics.py
+++ b/scripts/sysmon/net_metrics.py
@@ -12,6 +12,7 @@
 import socket
 
 import psutil
+from infra_libs import ts_mon
 
 from chromite.lib import cros_logging as logging
 from chromite.lib import metrics
@@ -85,7 +86,7 @@
                    fields=dict(direction='up', **fields))
         metric.set(getattr(counters, down_counter_name),
                    fields=dict(direction='down', **fields))
-      except metrics.MonitoringDecreasingValueError as ex:
+      except ts_mon.MonitoringDecreasingValueError as ex:
         # This normally shouldn't happen, but might if the network
         # driver module is reloaded, so log an error and continue
         # instead of raising an exception.