Lambda PassThrough trace header support

[majanjua-amzn]

Lambda trace header with passthrough will be missing parent and sampled attributes, we should be able to support this.

Description of changes:

• Detect lambda trace context and mark segments with noOp attribute, signifying passthrough mode
• Propagate this attribute to subsegments when they are added
• Detect noOp attribute to stop trace context propagation in passthrough mode
• Unit test checking if segment generated with passthrough behaviour (segment.noOp == true) results in the trace header not being propagated as expected

Testing:
Tested appropriate behaviour using two lambda functions:

• Lambda A (passthrough) calls Lambda B (active): [x]
• Lambda A (active) calls Lambda B (active):

And tested combinations of the following:

• Lambda function with old passthrough behaviour
• Lambda function with new passthrough behaviour
• Lambda function with AWS SDK v2
• Lambda function with AWS SDK v3

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.57%. Comparing base (1502470) to head (5cc208c).

❗ Current head 5cc208c differs from pull request most recent head dd12415

Please upload reports for the commit dd12415 to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #660      +/-   ##
==========================================
+ Coverage   83.49%   83.57%   +0.07%     
==========================================
  Files          37       37              
  Lines        1806     1814       +8     
==========================================
+ Hits         1508     1516       +8     
  Misses        298      298              

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[wangzlei]

LGTM

[jj22ee]

There is one more patcher (fetch) that is introduced in sdk_contrib folder, can we confirm if your changes need to be applied to this one as well.
https://github.com/aws/aws-xray-sdk-node/blob/master/sdk_contrib/fetch/lib/fetch_p.js#L107-L110

[jj22ee]

Thanks!