Updated Rack/Rails span error setting behavior

[ericmustin]

This PR addresses #1155 , when a user modifies or handles a Rails Controller exception in a rescue_from block. Since adding #1124, the instrumentation has been setting any rails errors upstream on the rack span. However, in cases where these exceptions get swallowed, or re-raised with a different exception, in a rescue_from, this causes the rack span to contain stale information, since we haven't been accounting for rescue_from in our current rails instrumentation.

By moving the rails => rack span upstream error propagation from the action_controller instrumentation, and into the datadog rails exception_middleware, we ensure we're modifying the rack span as late as possible before the exception information is swallowed by rails internal exception handling. This means that we are not going to incorrectly set a handled exception on the rack span.

Long term, adding more explicit instrumentation of the rescue_from code blocks (perhaps by patching ActionController::Rescue.process_action) and Controller before/after hooks would also cover the above use case, but it's a little unclear what that would look like (are they child spans of the controller? part of the controller? child spans of rack? etc) and how to safely test and handle all the edge cases that come with that in a way we could release reliably. For now, the priority of this PR is to ensure the rack spans are not receiving stale error information.

[marcotc]

👍