Add AWS/SecretsManager to the services list

At the moment `AWS/SecretsManager` namespace has only metric in there - `ResourceCount`, it can be used to keep track of count of secrets in the region (secrets count is not being reported to the `AWS/Usage` namespace for some reason)

- https://docs.aws.amazon.com/secretsmanager/latest/userguide/monitoring-cloudwatch.html

pkg/config/services.go

@@ -720,6 +720,10 @@ var SupportedServices = serviceConfigs{
 			regexp.MustCompile("(?P<BucketName>[^:]+)$"),
 		},
 	},
+	{
+		Namespace: "AWS/SecretsManager",
+		Alias:     "secretsmanager",
+	},
 	{
 		Namespace: "AWS/SES",
 		Alias:     "ses",