it turned out that some of the tests were leaky and didn't catch that ExceptionUtils were throwing an exception (race condition with Hystrix). That was due to the fact that When Hystrix with Feign were doing retries the RequestInterceptor wasn't called. That means that a new span wasn't created but a parent span was closed.
With this change the only place where the span creation and closing takes place is TraceFeignClient. I removed the Feign RequestInterceptor. Now whenever there is a retry - a new span is created and closed after getting a response. There are no exceptions, special cases etc.
In addition to that since Feign is fully immutable and SpanInjector is by design made to mutate objects I had to wrap the immutable Request in an AtomicReference in order to change the contents of the Request. I'm ashamed but didn't have a better idea. Since that is packaged scope nobody should every see that (outside the package of course)
9ebd9184ca