trace: tag http.route on api-gateway spans

[Julio-Guerra]

What does this PR do?

Add the missing http.route tag to api gateway spans.

Motivation

Support the APM's API Catalog which uses this span tag to list the entries of the API Catalog.

Testing Guidelines

TODO - this is just a draft PR to bootstrap the work and help relevant teams

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)