cold start tracing (python)

[joeyzhao2018]

What does this PR do?

Add Cold Start Tracing feature to Datadog lambda python layer.
This feature is already available in datadog-lambda-js layer and this is its PR link.

Motivation

Showing the spans to indicate the time consumed by loading each modules can help users better understand the cold start causes.

Testing Guidelines

Deployed in a testing lambda.

1. Check the spans show up as expected.
2. Invoke it multiple times to see non-cold-start cases.
3. Tested with the inferred-spans-python setup, which has one lambda triggering following lambdas via SQS/SNS etc.
4. Change the env variables to see they are working as expected.
   • with DD_COLD_START_TRACING=false
   • with DD_MIN_COLD_START_DURATION=80
   • with DD_COLD_START_TRACE_SKIP_LIB=ddtrace.internal.sampling,jsonschema

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]

Great work on this Joey. I think overall it's ready to go, just two comments.

[astuyve]

Ship it!

[PierreKiwi]

Hello,
Is this feature documented anywhere ? Because it appears to be a significant change (not sure about the billing aspect).

[astuyve]

Hi @PierreKiwi - there's a blog post here and environment variables controlling the granularity of this feature are documented here for now, which will expand as we roll out support to new runtimes and integrations.

Also if you have specific questions, feel free to ping me in our open slack: https://chat.datadoghq.com/

[PierreKiwi]

Thanks @astuyve !
I saw the blog so I connected the dots eventually. I was googling DD_COLD_START_TRACING but it did not lead me anywhere :)
Also, I am using the CDK construct v2 where the new options are not yet available.

[astuyve]

No problem, it's great to hear from our users either way!

Sorry for that, I'm working with our technical writers to further document these features more centrally. And yes, CDK is next on my list.

If you've got specific questions, definitely reach out in slack and I'm happy to help answer anything specifically.

I hope this helps you!