Feature - Request Timelines

[cnoon]

These changes started with me experimenting with the ideas in #1029, then became something pretty awesome!

This PR adds the ability for Alamofire to track the timings of a Request throughout it's entire lifecycle with virtually zero overhead. The Timeline object stores all the timing info about the Request and is delivered as part of the Response object. The timings are collected at various points throughout the lifecycle of the request.

Alamofire can now report the latency, request duration and total duration of any Request. I also added CustomStringConvertible and CustomDebugStringConvertible conformance to the Timeline to make it really easy to print out the info.

[cnoon]

cc @jshier, @kcharwood, @kylef for review

[cnoon]

I'll add a section to the README about this once everyone is on board that this is a good set of changes to put in.

[jshier]

There's a new line between this and the [Result] output. Perhaps there's an extra newline in result.debugDescription? Trivial, but didn't quite match the rest of the output.

[cnoon]

Ah bummer. I'll check it out.

[cnoon]

This is possible if the value stored in the result contains newlines. There's nothing to fix here.

[alimoeeny]

Wow Dude @cnoon , this is much more complete than what I had in mind, thank you.
I need to try it and see how I can collect all the metrics (timelines I guess) in one location to log or send back to a log aggregator.

[cnoon]

Thanks @alimoeeny! It was definitely fun to put together.

[jshier]

👍

@alimoeeny I do logging in my custom .responseObject extension, taking the closure that comes in, adding my logging call, and passing the result the actual serializer. I imagine you could do the same, especially if you have a custom response closure already.

[alimoeeny]

thanks @jshier I think I'd do the same.