Prevent S3 URL presigning from generating a remote request span

[marcotc]

Fixes #922

This PR addresses ddtrace capturing a "fake" S3 request that the AWS S3 SDK reports when presigning a request URL.

Presigned URLs give users a pre-authenticated URL to be used in the future to retrieve data from S3, but they do not preform any data retrieval to get created.

The Ruby S3 SDK performs a "fake" request internally to generated a presigned URL: it creates an S3 get_object request, sends it through the regular request handler (one of which being Datadog::Contrib::Aws::Handler), but hijacks the "data send" step at the end by not actually sending the request.

Unfortunately, there's no data we can use to differentiate between a real request and a fake request in our ddtrace handler. I've opened a PR with AWS to provide us with that information, so we can simplify the logic from this PR in the future: aws/aws-sdk-ruby#2513

This PR effectively removes our request handler from these fake presign requests, the same approach taken internally by the S3 SDK for some of its own handlers that do not play nice with the fake presign request: https://github.com/aws/aws-sdk-ruby/blob/656691b470bb970627286cf1df2bd84049b6787d/gems/aws-sdk-s3/lib/aws-sdk-s3/presigner.rb#L194-L196

[allcentury]

@marcotc congrats on getting the AWS SDK updated as well, this looks great. We're fine with this solution, or one where a AWS SDK minimum version is required.

[ericmustin]

I think this is good to go, one small nit in code comments, not blocking though.

It looks like your upstream PR got merged but aws doesn't cut a new release that frequently, however in the future we may want to revist this and rely on the upstream approach of checking context[:presigned_url] for relevant versions.

[marcotc]

It looks like your upstream PR got merged but aws doesn't cut a new release that frequently, however in the future we may want to revist this and rely on the upstream approach of checking context[:presigned_url] for relevant versions.

It would be nice.
We still have to support clients with older versions though...

I'll add a comment referencing the easier way to do it in the code.