Provide a -usage flag for printing the usage of a single plugin

Closes #136

cmd/telegraf/telegraf.go

@@ -13,13 +13,18 @@ import (
 	_ "github.com/influxdb/telegraf/plugins/all"
 )
 
-var fDebug = flag.Bool("debug", false, "show metrics as they're generated to stdout")
+var fDebug = flag.Bool("debug", false,
+	"show metrics as they're generated to stdout")
 var fTest = flag.Bool("test", false, "gather metrics, print them out, and exit")
 var fConfig = flag.String("config", "", "configuration file to load")
 var fVersion = flag.Bool("version", false, "display the version")
-var fSampleConfig = flag.Bool("sample-config", false, "print out full sample configuration")
+var fSampleConfig = flag.Bool("sample-config", false,
+	"print out full sample configuration")
 var fPidfile = flag.String("pidfile", "", "file to write our pid to")
-var fPLuginsFilter = flag.String("filter", "", "filter the plugins to enable, separator is :")
+var fPLuginsFilter = flag.String("filter", "",
+	"filter the plugins to enable, separator is :")
+var fUsage = flag.String("usage", "",
+	"print usage for a plugin, ie, 'telegraf -usage mysql'")
 
 // Telegraf version
 //	-ldflags "-X main.Version=`git describe --always --tags`"
@@ -39,6 +44,13 @@ func main() {
 		return
 	}
 
+	if *fUsage != "" {
+		if err := telegraf.PrintPluginConfig(*fUsage); err != nil {
+			log.Fatal(err)
+		}
+		return
+	}
+
 	var (
 		config *telegraf.Config
 		err    error

config.go

@@ -421,3 +421,15 @@ func PrintSampleConfig() {
 		}
 	}
 }
+
+// PrintPluginConfig prints the config usage of a single plugin.
+func PrintPluginConfig(name string) error {
+	if creator, ok := plugins.Plugins[name]; ok {
+		plugin := creator()
+		fmt.Printf("# %s\n[%s]\n", plugin.Description(), name)
+		fmt.Printf(strings.TrimSpace(plugin.SampleConfig()))
+	} else {
+		return errors.New(fmt.Sprintf("Plugin %s not found", name))
+	}
+	return nil
+}