Exclude loopback interface from ethtool (DataDog#13042)

* Exclude loopback interfaces lo and lo0 from SIOCETHTOOL

Loopback interfaces doesn't support this ioctl call and they
are not gonna expose driver specific informations. Exclude them
so that we don't generate error logs.

* Update configuration example to mention lo exclusion for ethtool

Author: bonnefoa

network/assets/configuration/spec.yaml

@@ -148,6 +148,8 @@ files:
               The queue and cpu will be used as a tag when submitting metrics.
               For example, 'queue_0_tx_cnt' will be submitted as 'system.net.ena.queue.tx_cnt' with tag 'queue:0'
 
+              Loopback interfaces 'lo' and 'lo0' are excluded as they don't support SIOCETHTOOL.
+
               Currently supported drivers:
               - ena
               - virtio_net

network/datadog_checks/network/data/conf.yaml.default

@@ -126,6 +126,8 @@ instances:
     ## The queue and cpu will be used as a tag when submitting metrics.
     ## For example, 'queue_0_tx_cnt' will be submitted as 'system.net.ena.queue.tx_cnt' with tag 'queue:0'
     ##
+    ## Loopback interfaces 'lo' and 'lo0' are excluded as they don't support SIOCETHTOOL.
+    ##
     ## Currently supported drivers:
     ## - ena
     ## - virtio_net

network/datadog_checks/network/network.py

@@ -1064,6 +1064,10 @@ def _handle_ethtool_stats(self, iface, custom_tags):
         if iface in self._excluded_ifaces or (self._exclude_iface_re and self._exclude_iface_re.match(iface)):
             # Skip this network interface.
             return
+        if iface in ['lo', 'lo0']:
+            # Skip loopback ifaces as they don't support SIOCETHTOOL
+            return
+
         driver_name, driver_version, ethtool_stats_names, ethtool_stats = self._fetch_ethtool_stats(iface)
         tags = [] if custom_tags is None else custom_tags[:]
         tags.append('driver_name:{}'.format(driver_name))