Support extration of B3 headers

[stroem]

What does this PR do?

This will use dd-trace-go/tracer propagation extract to parse the B3 header in a specfic order by the environment variable

• Update unmarshalEventForTraceContext(..) to use propagator.Extract to populate traceCtx
• Added test for B3 metadata

Issue to this ticket https://github.com/DataDog/datadog-lambda-go/issues/50

Motivation

We have a service running OpenCensus and wants to be able to use ochttp.Transport to pass B3 headers to the dd_lambda-Wrapper

Additional Notes

Due to the propagator is created in init-scope and NewPropagator is parsing the environment variable in New-constructor, I have added a requirement to be able to run the test: you need to specify the DD_PROPAGATION_STYLE_EXTRACT before running that particular test case

Types of changes

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

Checklist

• [x] 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

[nhinsch]

Hi @stroem, thanks for submitting this! This looks potentially useful, but unfortunately we are not going to merge this PR because it would conflict with a different approach for extracting trace/span context from custom headers using a customizable trace extractor function that we've already implemented in the JavaScript library in this PR: https://github.com/DataDog/datadog-lambda-js/pull/151. If we were to enable alternative ways to extract trace headers in the Go library, we would want to match the behavior of the JavaScript library as closely as possible to maintain consistency.