Add support for standard proxy env vars in outputs. (#3212)

(cherry picked from commit 7b08f9d099)

plugins/outputs/amon/amon.go

@@ -48,6 +48,9 @@ func (a *Amon) Connect() error {
 		return fmt.Errorf("serverkey and amon_instance are required fields for amon output")
 	}
 	a.client = &http.Client{
+		Transport: &http.Transport{
+			Proxy: http.ProxyFromEnvironment,
+		},
 		Timeout: a.Timeout.Duration,
 	}
 	return nil