Simplify aws-sdk traces

[mcculls]

What Does This Do

This PR removes the underlying HTTP client spans beneath AWS-SDK calls. The HTTP client span is an implementation detail of the AWS-SDK call, rather than a request to a different service, and collapsing the trace to just show the AWS-SDK calls makes distributed traces easier to follow.

Users who require the old behaviour can re-enable it with this system property:

-Ddd.aws-sdk.legacy.tracing.enabled=true

or by setting this environment variable:

DD_AWS_SDK_LEGACY_TRACING_ENABLED=true

Motivation

We want to make SQS receive spans more consistent with other messaging integrations. This will involve excluding certain SQS calls from our AWS-SDK instrumentation and adding new SQS specific instrumentation. The new SQS instrumentation will avoid creating spans when no messages are received, but underneath the AWS-SDK is still making an HTTP call to check for new messages. Since we don't know at the time of the underlying HTTP call whether we will actually get any messages we cannot filter out any HTTP span at that point. It is also hard to tell which AWS service is being called without examining the attached entity since AWS-SDK uses POST by default.

The simplest solution was to remove the underlying HTTP client spans beneath AWS-SDK calls.

Before:

After:

Additional Notes

This PR also moves setting of the X-Ray propagation header out from the different low-level HTTP instrumentations and into the AWS-SDK instrumentations where it really belongs. (To avoid adding an extra helper class the propagation setter interface was implemented by the existing AWS-SDK decorators.)

[ygree]

lgtm