feat(aap): update tags on inferred spans for API Gateway

[florentinl]

What does this PR do?

It updates tags for API Gateway Rest Proxy and HTTP API inferred spans, to enable discovery by both the APM Endpoint Catalog and the API Gateway Catalog:

From the RFC:

Motivation

This PR implements RFC-1081.

The goal is be able to use API Gateway lambda inferred spans for endpoint discovery and correlation in the AppSec API Catalog.

Testing Guidelines

• unit tests have been updated to reflect the expected behavior
• snapshot tests: updated
• system-tests done: http://github.com/DataDog/system-tests/pull/5830

Additional Notes

• _dd.appsec.* meta and metrics are to be set in ddtrace: feat(aap): report AAP on lambda inferred spans dd-trace-py#15511
• renamed http.user_agent into http.useragent, this is the tag specified in the "Span attributes" page of APM ( and the tag is used by the system-tests to correlate spans with requests, this is necessary to test this feature there)
• added the _dd.inferred_span metric, the backend filters inferred spans for the APM Catalog (and the AAP Catalog) based on this metric. It is already set by tracers for the API Gateway proxy inferred spans.

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)