HAR export support

[atdt]

There exists a very handy tool for performance analysis called BrowserMob Proxy. It allows you to capture HAR files for any browser by acting as a simple MITM proxy. It does not support SPDY, HTTP 2, or TLS, though. Proxygen could be an ideal replacement, if it had the ability of recording requests and outputting them as HAR. I see that there already exists code for reading / replaying HAR files in the test suite code, so I'm wondering if recording HAR files is on the horizon.

[afrind]

Given that this needs to be a proxy to be really useful, we'd have to do this after making more of our proxy implementations open source. It could be more simply implemented as an HTTP server filter, but it's not on our immediate priority list. Want to send a PR?