diff --git a/.changelog/12882.txt b/.changelog/12882.txt
new file mode 100644
index 00000000000..f4981fa8cfd
--- /dev/null
+++ b/.changelog/12882.txt
@@ -0,0 +1,3 @@
+```release-note:enhancement
+monitoring: added more description to make it clear that label starting with "__" are reserved for internal use case only in metrics labels, and cannot be overwritten. 
+```
\ No newline at end of file
diff --git a/google/services/monitoring/resource_monitoring_alert_policy.go b/google/services/monitoring/resource_monitoring_alert_policy.go
index 73e0b4f7a75..356cdc6ebed 100644
--- a/google/services/monitoring/resource_monitoring_alert_policy.go
+++ b/google/services/monitoring/resource_monitoring_alert_policy.go
@@ -432,9 +432,10 @@ rule group.`,
 must be valid.
 
 Label values can be templatized by using variables. The only available
-variable names are the names of the labels in the PromQL result, including
-"__name__" and "value". "labels" may be empty. This field is intended to be
-used for organizing and identifying the AlertPolicy`,
+variable names are the names of the labels in the PromQL result,
+although label names beginning with \_\_ (two "\_") are reserved for
+internal use. "labels" may be empty. This field is intended to be used
+for organizing and identifying the AlertPolicy.`,
 										Elem: &schema.Schema{Type: schema.TypeString},
 									},
 									"rule_group": {
diff --git a/website/docs/r/monitoring_alert_policy.html.markdown b/website/docs/r/monitoring_alert_policy.html.markdown
index c39aac4a103..9202e01e994 100644
--- a/website/docs/r/monitoring_alert_policy.html.markdown
+++ b/website/docs/r/monitoring_alert_policy.html.markdown
@@ -807,9 +807,10 @@ The following arguments are supported:
   Labels to add to or overwrite in the PromQL query result. Label names
   must be valid.
   Label values can be templatized by using variables. The only available
-  variable names are the names of the labels in the PromQL result, including
-  "__name__" and "value". "labels" may be empty. This field is intended to be
-  used for organizing and identifying the AlertPolicy
+  variable names are the names of the labels in the PromQL result,
+  although label names beginning with \_\_ (two "\_") are reserved for
+  internal use. "labels" may be empty. This field is intended to be used
+  for organizing and identifying the AlertPolicy.
 
 * `rule_group` -
   (Optional)