feat(instrumentation-aws-lambda): Adds lambdaHandler config option

[schmalzs]

Which problem is this PR solving?

This PR is scoped to the @opentelemetry/instrumentation-aws-lambda package.

Currently, the AWS Lambda instrumentation uses the _HANDLER environment variable to determine which module to instrument. The _HANDLER environment variable is set by AWS itself and, for most use cases, will accurately represent the handler that should be targeted by this instrumentation.

There exist use cases where the _HANDLER environment variable does not accurately represent the module that should be targeted by this instrumentation.

Specifically, I have Lambdas that are integrated with Datadog. The Datadog integration injects itself into the Lambda runtime by overriding the _HANDLER environment variable. It points the _HANDLER to Datadog code (loaded via a Lambda Layer). That Datadog code acts as a proxy that wraps my actual Lambda handler code.

When attempting to use this instrumentation alongside the Datadog integration, the instrumentation does not work. It ends up targeting the Datadog code (due to the value of the _HANDLER environment variable) rather than my actual Lambda code.

My particular use case is specific to utilizing this instrumentation alongside Datadog, however, I imagine there are other use cases that would run into similar problems.

Short description of the changes

This PR adds an optional handler config option. This option accepts a string that can be used to manually specify the handler that the instrumentation should target. If the handler option is omitted, the default behavior will be to use the _HANDLER environment variable, as it does today.

I have tested this alongside the Datadog integration and have confirmed it works as expected.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: schmalzs / name: Scott Schmalz (8b7a5b1, 83f1e59, b2a19c5, 474bf6b)
• ✅ login: KivSee / name: Amir Blum (83f1e59)
• ✅ login: blumamir / name: Amir Blum (bec3509)

[pichlermarc]

cc @carolabadeer (Component Owner)

[codecov]

Codecov Report

Merging #1627 (bec3509) into main (825b5a8) will decrease coverage by 0.02%.
The diff coverage is 66.66%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1627      +/-   ##
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- Coverage   91.78%   91.77%   -0.02%     
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  Files         139      139              
  Lines        7110     7112       +2     
  Branches     1426     1427       +1     
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+ Hits         6526     6527       +1     
- Misses        584      585       +1     

Files Changed	Coverage Δ
...-instrumentation-aws-lambda/src/instrumentation.ts	93.67% <66.66%> (-0.51%)	⬇️

[blumamir]

Thanks for the great research and for adding this feature

[blumamir]

Thanks for addressing the comments 💯
Left a few nit suggestions

[blumamir]

LGTM

[blumamir]

Will merge after @carolabadeer review as component owner

[blumamir]

@schmalzs seems that lint is failing in CI.
Can you please run npm run lint:fix for this package and push the changes? thanks

[schmalzs]

@schmalzs seems that lint is failing in CI. Can you please run npm run lint:fix for this package and push the changes? thanks

Just updated the linting fixes ✅

Thanks for your quick review of this PR, @blumamir, and all your thoughtful feedback. Really appreciate it!

[carolabadeer]

Left a few small nit comments, otherwise changes look good to me! Thanks @schmalzs for this change and your attention to detail, and thanks @blumamir for your review :)

[schmalzs]

Thank you for your thoughtful review, @carolabadeer! I just made the updates you suggested. Please let me know what you think. Thanks!

[carolabadeer]

LGTM