Add validation to CloudWatch metric query name

[cab105]

For context, the metric query name is used by the source to populate the ID field used to identify the query with CloudWatch for follow-up queries and manipulation. The name must adhere to the following regexp as per AWS (https://docs.aws.amazon.com/AmazonCloudWatch/latest/APIReference/API_MetricDataQuery.html):

[a-z][a-zA-Z0-9_]*

[antoineco]

Tip: the second part can we shortened to \w*.

[cab105]

Doesn't \w include more than alphanumeric?

[antoineco]

Regex101.com says

\w matches any word character (equivalent to [a-zA-Z0-9_])

I wouldn't have known without it to be honest 😄

[cab105]

Wow my memory is getting rusty:
https://media.giphy.com/media/83QtfwKWdmSEo/giphy.gif