[SVLS-4422] Support for metrics with timestamps when the Extension is present

[DylanLovesCoffee]

What does this PR do?

Adds support for metrics with timestamps (AKA historical metrics) when using the Extension. Dogstatsd does not support historical distribution metrics yet, so we must use the Datadog API client.

We were lazy loading the DD API module when the Extension was not present, so I've tried to be careful not to undo that work. Even with these changes, we should only initialize the API client once per Lambda environment if a timestamp parameter is detected when using lambda_metric.

Flame graph of imported modules on a cold start:

Motivation

#398

Testing Guidelines

Added a simple unit test + manual testing.

Additional Notes

Sister PR for NodeJS: DataDog/datadog-lambda-js#522

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)

[duncanista]

Amazing work Dylan – thanks for lazy loading and improving the code!

Could we add a screenshot to the PR showing a flamegraph with the overhead implications of this change for customers using the extension + timestamp metrics?

[DylanLovesCoffee]

Dismissing reviews for now since I'm running into an issue with this this PR where all spans in a cold start have different trace IDs :( will tackle after the weekend

[joeyzhao2018]

LGTM