Show platform not supported warning only on plugin creation (#6801)

plugins/inputs/ethtool/ethtool.go

@@ -2,6 +2,8 @@ package ethtool
 
 import (
 	"net"
+
+	"github.com/influxdata/telegraf"
 )
 
 type Command interface {
@@ -18,6 +20,8 @@ type Ethtool struct {
 	// This is the list of interface names to ignore
 	InterfaceExclude []string `toml:"interface_exclude"`
 
+	Log telegraf.Logger `toml:"-"`
+
 	// the ethtool command
 	command Command
 }

plugins/inputs/ethtool/ethtool_notlinux.go

@@ -3,19 +3,21 @@
 package ethtool
 
 import (
-	"log"
-
 	"github.com/influxdata/telegraf"
 	"github.com/influxdata/telegraf/plugins/inputs"
 )
 
+func (e *Ethtool) Init() error {
+	e.Log.Warn("Current platform is not supported")
+	return nil
+}
+
 func (e *Ethtool) Gather(acc telegraf.Accumulator) error {
 	return nil
 }
 
 func init() {
 	inputs.Add(pluginName, func() telegraf.Input {
-		log.Print("W! [inputs.ethtool] Current platform is not supported")
 		return &Ethtool{}
 	})
 }