Change duration -> internal and implement private gopsutil methods

outputs/librato/librato.go

@@ -8,7 +8,7 @@ import (
 	"net/http"
 
 	"github.com/influxdb/influxdb/client/v2"
-	"github.com/influxdb/telegraf/duration"
+	"github.com/influxdb/telegraf/internal"
 	"github.com/influxdb/telegraf/outputs"
 )
 
@@ -16,7 +16,7 @@ type Librato struct {
 	ApiUser   string
 	ApiToken  string
 	SourceTag string
-	Timeout   duration.Duration
+	Timeout   internal.Duration
 
 	apiUrl string
 	client *http.Client