feat: Add the RDS performance and generic metrics option

Author: ZPascal

plugins/inputs/aliyuncms/README.md

@@ -62,6 +62,12 @@ See the [CONFIGURATION.md][CONFIGURATION.md] for more details.
   # public_key_id = ""
   # role_name = ""
 
+  ## Aliyun Metric services
+  ## Specified which metric services to capture from Aliyun, choose from:
+  ##  * cms - Cloud Monitor service (default settings)
+  ##  * rds - Relational Database service
+  # metric_services = ["cms"]
+
   ## Specify ali cloud regions to be queried for metric and object discovery
   ## If not set, all supported regions (see below) would be covered, it can
   ## provide a significant load on API, so the recommendation here is to