Update Instrumenting Amazon API Gateway with v2 content

[domalessi]

What does this PR do? What is the motivation?

https://datadoghq.atlassian.net/browse/DOCS-10997

Updating the Instrumenting Amazon API Gateway page to incorporate new content, as introduced in this PR: DataDog/datadog-cdk-constructs#430.

Merge instructions

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• Ready for merge

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Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/domalessi/docs-10997-apigateway/tracing/trace_collection/proxy_setup/apigateway

[github-actions]

✅ Documentation Team Review

The documentation team has approved this pull request. Thank you for your contribution!

[purple4reina]

Can we also include here info on what customers will need to do this manually, should they not deploy using the cdk? I'm thinking something along the lines of the "header|value" table that's included in V1. You can see what exactly those are here.

It's vitally important that the x-dd-proxy-request-time-ms gets three zeros appended, as done with "${context.requestTimeEpoch}000", which turns the request time from seconds to milliseconds. Without this, the start time of the synthetic span will be off my a factor of 1000.

[purple4reina]

These headers are set differently based on apigw version. Specifically the requestTimeEpoch needs to be handled differently in v2 than in v1.

Take a look at https://github.com/DataDog/datadog-cdk-constructs/blob/b7a9aa75a8d40a89718481fecf8f41e175f5704f/src/apigateway/parameters.ts for the different values that should be set based on v1 or v2.