APMSVLS-65 Extract Trace Context For AppSync Events

[rithikanarayan]

What does this PR do?

This PR adds logic to extract the trace context of an event that comes from AppSync for Python apps. This logic is placed in the helper function extract_context_from_appsync_event in tracing.py, modeled after other extractor functions for different services like SQS Queues or Kinesis. The new helper function is called in extract_dd_trace_context in the case that the even source is AppSync.

Motivation

Currently, if a customer has a setup where they use RUM to start a trace that goes through AppSync and triggers a Lambda function, we ask them to write a custom function to extract the trace context from the AppSync event that is passed to the Lambda. See an example of such a function here. It would be simpler for customers and for ourselves to extract the trace context in the lambda tracer layers ourselves, as we do for invocations from other sources (ex. SQS, API Gateway, etc.). This PR does so for the Python tracer.

Testing Guidelines

Additional Notes

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

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