CloudWatch: Fix alerting for queries with Id (using GetMetricData) (#17899)

This commit addresses half of #13749 by making sure GetMetricData 
works for alerting. Math Expressions (compound metrics) will still not 
work for alerting, this would require a bigger refactoring of Grafana's 
alerting service. However, with this commit at least alerting for basic 
metrics with non empty query Id will work.

Fixes half of #13749

docs/sources/features/datasources/cloudwatch.md

@@ -60,6 +60,7 @@ Here is a minimal policy example:
             "Sid": "AllowReadingMetricsFromCloudWatch",
             "Effect": "Allow",
             "Action": [
+                "cloudwatch:DescribeAlarmsForMetric",
                 "cloudwatch:ListMetrics",
                 "cloudwatch:GetMetricStatistics",
                 "cloudwatch:GetMetricData"