feat: support custom metrics when using extension

[duncanista]

What does this PR do?

Allows custom metrics to be sent through the Datadog Extension.

Motivation

Custom metrics were only supported when using the Datadog Lambda Forwarder.

Testing Guidelines

• Added unit tests for metrics
• Tested manually on an AWS Lambda in both runtimes.

Additional Notes

• Updated dependencies to include dogstatsd-ruby a DogStatsD client for Ruby.

Types of changes

• [ ] Bug fix
• [x] New feature
• [ ] Breaking change
• [ ] 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