Added description for the add_mode processor

Signed-off-by: Robert Castley <[email protected]>

Author: rcastley

cmd/otelcol/config/collector/gateway_config.yaml

@@ -82,6 +82,7 @@ processors:
         #value: staging/production/...
         #key: deployment.environment
 
+  # The following processor is used to add "otelcol.service.mode" attribute to the internal metrics
   resource/add_mode:
     attributes:
       - action: insert