net/http: add GODEBUG=http1debug

[bradfitz]

The GODEBUG knob for http2 debugging (e.g. GODEBUG=http2debug=2) has proven to be very useful and popular.

Unfortunately it only works for HTTP/2.

There is code in the net/http package to do a similar thing for HTTP/1, but it's not accessible via an environment variable. Should we make it accessible with GODEBUG=http1debug=...?

[rsc]

Seems OK to me (Go1.9Early)

[gopherbot]

CL https://golang.org/cl/36548 mentions this issue.

[Sajmani]

I've updated the commit message in my CL with sample debug output. This is a first cut; I'm happy to adjust the output, content, leveling, etc. as needed.

[bradfitz]

One of the nice things about http2 is that it has a per-request "streamid" which shows up on all request and response lines, making it easy to line things up. Unfortunately, the stream id is unique only to a connection, though, so it's not a unique identifier for debugging, but generally works.

What I'd like to see for http1 is for each conn to get its own incrementing number, and each request on a conn to also get its own incrementing number, and then make sure each line about data going out or in for a request says either the relevant conn=%d or conn=%d,req=%d pair.

[Sajmani]

Doc for output discussion: https://docs.google.com/document/d/10FdNIgIjIyv9zfwrC7jSfdgjuWAHs6MdIrqaiz32vJU/edit#