[sysmon] Extract _is_virtual_netif function

BUG=None
TEST=None

Change-Id: I605f58b6a4ea0f87b6537c59e5074ea53f925363
Reviewed-on: https://chromium-review.googlesource.com/448805
Commit-Ready: Allen Li <ayatane@chromium.org>
Tested-by: Allen Li <ayatane@chromium.org>
Reviewed-by: Allen Li <ayatane@chromium.org>
diff --git a/scripts/sysmon/system_metrics.py b/scripts/sysmon/system_metrics.py
index e1f39c6..e9453ff 100644
--- a/scripts/sysmon/system_metrics.py
+++ b/scripts/sysmon/system_metrics.py
@@ -229,9 +229,7 @@
 
   nics = psutil.net_io_counters(pernic=True)
   for nic, counters in nics.iteritems():
-    # TODO(ayatane): Use a different way of identifying virtual interfaces
-    if nic.startswith('veth'):
-      # Skip virtual interfaces
+    if _is_virtual_netif(nic):
       continue
     fields = {'interface': nic}
     for metric, counter_name in metric_counter_names:
@@ -244,6 +242,12 @@
         logger.warning(str(ex))
 
 
+def _is_virtual_netif(nic):
+    """Return whether the network interface is virtual."""
+    # TODO(ayatane): Use a different way of identifying virtual interfaces
+    return nic.startswith('veth')
+
+
 def collect_proc_info():
   autoserv_count = 0
   sysmon_count = 0