Add `DbClusterIdentifier` to CloudWatch dimensions (#9297)

Unfortunately CloudWatch dimensions are case-sensitive and it uses both `DBClusterIdentifier` and `DbClusterIdentifier` (notice the lower case `b`) depending on the metric. All metrics which also have the `Role` dimension appear to use `DBClusterIdentifier`, whereas metric with the `EngineName` dimension use `DbClusterIdentifier`.

pkg/api/cloudwatch/metrics.go

@@ -126,7 +126,7 @@ func init() {
 		"AWS/NATGateway":       {"NatGatewayId"},
 		"AWS/OpsWorks":         {"StackId", "LayerId", "InstanceId"},
 		"AWS/Redshift":         {"NodeID", "ClusterIdentifier"},
-		"AWS/RDS":              {"DBInstanceIdentifier", "DBClusterIdentifier", "DatabaseClass", "EngineName", "Role"},
+		"AWS/RDS":              {"DBInstanceIdentifier", "DBClusterIdentifier", "DbClusterIdentifier", "DatabaseClass", "EngineName", "Role"},
 		"AWS/Route53":          {"HealthCheckId", "Region"},
 		"AWS/S3":               {"BucketName", "StorageType", "FilterId"},
 		"AWS/SES":              {},