S3 Downstream Span Pointers

[nhulston]

What does this PR do?

Adds span pointers to spans for Lambdas triggered by S3 putObject, copyObject, and completeMultipartUpload events. This change will affect the downstream case for Universal Instrumentation Lambda runtimes (Java, .NET, Golang).

Span pointers are similar to Span Links, but for cases when it is impossible to pass the Trace ID and Span ID between the spans that need to be linked.

When the calculated hashes for the upstream and downstream lambdas match, the Datadog backend will automatically link the two traces together.

IMAGES WILL GO HERE
When clicking on the linked span, a new tab opens linking to the downstream Lambda function that was triggered by this S3 object update.

Describe how you validated your changes

Mostly manual testing, but I also added unit tests.

To verify my changes, there are two ways.

Easy: Checkout this span.

More thorough: Run this Lambda function (only on the serverless AWS account) with the event payload

{
  "shouldPutObject": false,
  "shouldCopyObject": false,
  "shouldMultipartUpload": false
}

and change one of the bools to true. Check Datadog to ensure that the spans are properly linked.

I also added unit tests:

cargo test test_generate_span_pointer_hash
cargo test test_get_span_pointers
cargo test test_span_pointer_processing

Additional Notes

• This handles the 'downstream' case, for Lambdas that are triggered S3 requests. I will have another PR in each tracer to handle the 'upstream' case. Java PR here
• S3 and Dynamo are supported in Python+Node. I will soon open another PR in this tracer to add support for Dynamo.