use context.aws_request_id for the inferredSpan's request_id tag

[joeyzhao2018]

What does this PR do?

use context.aws_request_id for the inferredSpan's request_id tag

Motivation

This is a bug fix. Our convention is to use the context's aws_request_id for the request_id here. Nodejs version reference

The http payload's request Id can still be available with captureLambdaPayload option

Testing Guidelines

Additional Notes

Types of Changes

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

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
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• This PR passes the integration tests (ask a Datadog member to run the tests)

[duncanista]

LGTM!