Report correct length for trace span

[adam-fowler]

The trace span in the TracingMiddleware should be the length it takes to return a response. Unfortunately because our Response body is a closure that will write the body at a later point and not the actual body we have not written the response body to the channel by the time we end the span in the TracingMiddleware.

To fix this I am manually starting and ending the span myself instead of using withSpan. The TracingMiddleware returns a response body that ends the span once the body has been written.

Another issue arises if I leave this as is though. Someone could drop the response body which was returned by the TracingMiddleware and the span.end would never be called. To get around this I have wrapped it in a small class which will call span.end() on deinit, if it hasn't already been called.

[adam-fowler]

@slashmo You might be interested in this

[Joannis]

'been trying to think of a better solution - don't think there is one.