[SLS-2141] Fix keyError issue when creating inferred span

[hghotra]

What does this PR do?

Currently, when creating an inferred span, we use the [] syntax to access keys within the event payload. Switching this to .get(key) will make sure there is no keyError and the tag is simply ignored.

Motivation

Customer reported issue with creating API Gateway inferred span when apiid is not defined in the request context.

Testing Guidelines

Unit test & integration test.

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
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)

[jcstorms1]

LGTM 👍