chore: refactor enhanced metrics lambda arn parsing

[duncanista]

What does this PR do?

Refactors code involving parsing for AWS Lambda ARN.

Motivation

Some methods being refactored here are going to be used later in tracing. So I decided to refactor everything related to those methods, making it more simpler and readable.

Testing Guidelines

Updated unit tests.

Additional Notes

n/a

Types of Changes

• [ ] Bug fix
• [ ] New feature
• [ ] Breaking change
• [x] Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• [ ] This PR's description is comprehensive
• [ ] This PR contains breaking changes that are documented in the description
• [ ] This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• [ ] This PR impacts documentation, and it has been updated (or a ticket has been logged)
• [ ] This PR's changes are covered by the automated tests
• [ ] This PR collects user input/sensitive content into Datadog
• [ ] This PR passes the integration tests (ask a Datadog member to run the tests)