AWS::CloudWatch::Alarm - MetricDataQuery missing Period field from docs and schema

The `MetricDataQuery` structure does not contain a `Period` field, according to the docs or the CloudFormation schema file:

CloudFormation MetricDataQuery: https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-cloudwatch-alarm-metricdataquery.html

This field should exist according to CloudWatch however:

CloudWatch MetricDataQuery: https://docs.aws.amazon.com/AmazonCloudWatch/latest/APIReference/API_MetricDataQuery.html

Passing the `Period` field in a CloudFormation template file DOES work, however we vend a tool (https://github.com/aws/aws-cdk) that parses and generates code based on the CloudFormation schema, and since the Period field is not in the schema it is effectively invisible and unusable to us.

This field is required in order to generate an Alarm based on a Contributor Insights math expression (which our users currently cannot do).

## 6. Category

CloudWatch, Alarms, Monitoring, Management & Governance