Adds support for directly querying cloudwatch metrics via the AWS SDK

[johnewing1]

---

Description

Replaces PR #2262 . Internal version of this functionality had diverged sufficiently from original PR that it is cleaner to start afresh

Requires a role with the ListMetrics and GetMetricData capabilities. Role can be supplied using either the AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY environment variables or by attaching it to the EC2 instance on which bosun runs.

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

How has this been tested?

• [x] CloudWatch query functionality had good unit test coverage
• [x] Has been used in production in Skyscanner for several years

Checklist:

• [x] This contribution follows the project's code of conduct
• [x] This contribution follows the project's contributing guidelines
• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules

[johnewing1]

@neilfordyce Now includes the parseDimensions fix

[neilfordyce]

Looking good. Nice work on the wildcard dimensions, they make cloudwatch much easier to work with.