fix: plugin_metadata not populating to the correct etcd key (#2012) (#2021)

* fix: plugin_metadata not populating to the correct etcd key

* fix url also

(cherry picked from commit 4d004e1)

Co-authored-by: zhuoyang <zhuoyang98@gmail.com>

Author: lingsamuel

pkg/apisix/plugin_metadata.go

@@ -178,8 +178,8 @@ type pluginMetadataMem struct {
 
 func newPluginMetadataMem(c *cluster) PluginMetadata {
 	return &pluginMetadataMem{
-		url:      c.baseURL + "/plugin_metadatas",
-		resource: "plugin_metadatas",
+		url:      c.baseURL + "/plugin_metadata",
+		resource: "plugin_metadata",
 		cluster:  c,
 	}
 }