::: cowboy_sub_protocol

The `cowboy_sub_protocol` behaviour defines the interface used
by modules that implement a protocol on top of HTTP.

:: Callbacks

: upgrade(Req, Env, Handler, Opts)
	-> {ok, Req, Env}
	| {suspend, Module, Function, Args}
	| {stop, Req}

Types:

* Req = cowboy_req:req()
* Env = env()
* Handler = module()
* Opts = any()
* Module = module()
* Function = atom()
* Args = [any()]
* StatusCode = cowboy:http_status()

Upgrade the protocol.

Please refer to the `cowboy_middleware` manual for a
description of the return values.