Add prometheus third party integration

[dgo-]

Description

In this pull request I added support for the third party prometheus integration.

Type of change:

• Bug fix (non-breaking change which fixes an issue)
• 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
• Documentation fix/enhancement

Required Checklist:

• I have signed the MongoDB CLA
• I have read the Terraform contribution guidelines
• I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code

Further comments

The change itself will not break something, but I had to upgrade go.mongodb.org/atlas to v0.16.0, because it introduces support for the prometheus integration.

[themantissa]

@dgo- appreciate the hard work you've put into this but we already completed this and it will be in our next release when we are done with others features (#706). Expect it in the near term. Thank you!