Extract API Key from Secrets Manager when using the metrics API client

[DylanLovesCoffee]

What does this PR do?

Respects DD_API_KEY_SECRET_ARN as an API key parameter when using the DD metrics API client since this logic was never added.

Motivation

#577 - permission errors are thrown for Secrets Manager when using sendDistributionMetricWithDate() and DD_API_KEY_SECRET_ARN.

Testing Guidelines

Adds a unit test for secrets manager

Additional Notes

SVLS-5679

This will likely add a bit of overhead due to having to load the Secrets Manager AWS SDK, but this logic is lazy loaded and only hit when a metric is sent to the DD metrics API (most often used to send metrics with a retro timestamp).

Types of Changes

• Bug fix
• 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
• This PR passes the integration tests (ask a Datadog member to run the tests)