Handle datetime parameter for historical custom metrics submitted to the API

[DylanLovesCoffee]

What does this PR do?

We should allow datetime parameters passed to lambda_metric() when using ThreadStats

Motivation

Came up in support issue where a type error was raised

Testing Guidelines

Unit test

Additional Notes

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)

[astuyve]

just fix ints/2-byte size diff and you're good