Add an interface:"all" tag to the net protocol counters

fixes #508

plugins/inputs/system/net.go

@@ -94,7 +94,10 @@ func (s *NetIOStats) Gather(acc inputs.Accumulator) error {
 			fields[name] = value
 		}
 	}
-	acc.AddFields("net", fields, nil)
+	tags := map[string]string{
+		"interface": "all",
+	}
+	acc.AddFields("net", fields, tags)
 
 	return nil
 }

plugins/inputs/system/net_test.go

@@ -80,7 +80,10 @@ func TestNetStats(t *testing.T) {
 		"udp_noports":     int64(892592),
 		"udp_indatagrams": int64(4655),
 	}
-	acc.AssertContainsTaggedFields(t, "net", fields2, make(map[string]string))
+	ntags = map[string]string{
+		"interface": "all",
+	}
+	acc.AssertContainsTaggedFields(t, "net", fields2, ntags)
 
 	acc.Points = nil