CloudWatch: Handle permissions error and update docs (#88524)

docs/sources/datasources/aws-cloudwatch/_index.md

@@ -123,6 +123,12 @@ You can attach these permissions to the IAM role or IAM user you configured in [
       "Effect": "Allow",
       "Action": "tag:GetResources",
       "Resource": "*"
+    },
+    {
+      "Sid": "AllowReadingResourceMetricsFromPerformanceInsights",
+      "Effect": "Allow",
+      "Action": "pi:GetResourceMetrics",
+      "Resource": "*"
     }
   ]
 }
@@ -182,6 +188,12 @@ You can attach these permissions to the IAM role or IAM user you configured in [
       ],
       "Resource": "*"
     },
+    {
+      "Sid": "AllowReadingResourceMetricsFromPerformanceInsights",
+      "Effect": "Allow",
+      "Action": "pi:GetResourceMetrics",
+      "Resource": "*"
+    },
     {
       "Sid": "AllowReadingLogsFromCloudWatch",
       "Effect": "Allow",