Introduce the req_filter Websocket option

This option allows customizing the compacting of the Req object
when using Websocket. By default it will keep most public fields
excluding headers of course, since those can be large.

src/cowboy_websocket.erl

@@ -47,14 +47,13 @@
 -callback websocket_info(any(), State)
 	-> call_result(State) when State::any().
 
-%% @todo OK this I am not sure what to do about it. We don't have a Req anymore.
-%% We probably should have a websocket_terminate instead.
 -callback terminate(any(), cowboy_req:req(), any()) -> ok.
 -optional_callbacks([terminate/3]).
 
 -type opts() :: #{
+	compress => boolean(),
 	idle_timeout => timeout(),
-	compress => boolean()
+	req_filter => fun((cowboy_req:req()) -> map())
 }.
 -export_type([opts/0]).
 
@@ -71,7 +70,8 @@
 	frag_state = undefined :: cow_ws:frag_state(),
 	frag_buffer = <<>> :: binary(),
 	utf8_state = 0 :: cow_ws:utf8_state(),
-	extensions = #{} :: map()
+	extensions = #{} :: map(),
+	req = #{} :: map()
 }).
 
 %% Stream process.
@@ -90,7 +90,11 @@ upgrade(Req, Env, Handler, HandlerState) ->
 upgrade(Req0, Env, Handler, HandlerState, Opts) ->
 	Timeout = maps:get(idle_timeout, Opts, 60000),
 	Compress = maps:get(compress, Opts, false),
-	State0 = #state{handler=Handler, timeout=Timeout, compress=Compress},
+	FilteredReq = case maps:get(req_filter, Opts, undefined) of
+		undefined -> maps:with([method, version, scheme, host, port, path, qs, peer], Req0);
+		FilterFun -> FilterFun(Req0)
+	end,
+	State0 = #state{handler=Handler, timeout=Timeout, compress=Compress, req=FilteredReq},
 	try websocket_upgrade(State0, Req0) of
 		{ok, State, Req} ->
 			websocket_handshake(State, Req, HandlerState, Env)
@@ -417,5 +421,5 @@ terminate(State, HandlerState, Reason) ->
 	handler_terminate(State, HandlerState, Reason),
 	exit(normal).
 
-handler_terminate(#state{handler=Handler}, HandlerState, Reason) ->
-	cowboy_handler:terminate(Reason, undefined, HandlerState, Handler).
+handler_terminate(#state{handler=Handler, req=Req}, HandlerState, Reason) ->
+	cowboy_handler:terminate(Reason, Req, HandlerState, Handler).