[META 745] Implement Azure Functions metadata detection

[elastic-apm-tech]

See meta issue for the description and details:

• Meta issue: Implement Azure Functions metadata detection apm#745

[trentm]

I have some follow-up questions on the Azure Functions metadata collection here: elastic/apm#712 (review)

This is based on the gherkin spec file that was added later in that PR.

[trentm]

My questions are answered well-enough. I think the changes in #3071 handle this.