Introduce undocumented option logger

This commit reworks the logging that Cowboy does via
error_logger to make the module that will do the actual
logging configurable.

The logger module interface must be the same as logger
and lager: a separate function per log level with the
same log levels they support.

The default behavior remains to call error_logger,
although some messages were downgraded to warnings
instead of errors. Since error_logger only supports
three different log levels, some messages may get
downgraded/upgraded depending on what the original
log level was to make them compatible with error_logger.

The {log, Level, Format, Args} command was also
added to stream handlers. Stream handlers should
use this command to log messages because it allows
writing a stream handler to intercept some of those
messages and extract information or block them as
necessary.

The logger option only applies to Cowboy itself,
not to the messages Ranch logs, so more work remains
to be done in that area.

src/cowboy.erl

@@ -19,6 +19,10 @@
 -export([stop_listener/1]).
 -export([set_env/3]).
 
+%% Internal.
+-export([log/2]).
+-export([log/4]).
+
 %% @todo Detailed opts.
 -type opts() :: map().
 -export_type([opts/0]).
@@ -71,3 +75,30 @@ set_env(Ref, Name, Value) ->
 	Env = maps:get(env, Opts, #{}),
 	Opts2 = maps:put(env, maps:put(Name, Value, Env), Opts),
 	ok = ranch:set_protocol_options(Ref, Opts2).
+
+%% Internal.
+
+-spec log({log, logger:level(), io:format(), list()}, opts()) -> ok.
+log({log, Level, Format, Args}, Opts) ->
+	log(Level, Format, Args, Opts).
+
+-spec log(logger:level(), io:format(), list(), opts()) -> ok.
+log(Level, Format, Args, #{logger := Logger})
+		when Logger =/= error_logger ->
+	_ = Logger:Level(Format, Args),
+	ok;
+%% We use error_logger by default. Because error_logger does
+%% not have all the levels we accept we have to do some
+%% mapping to error_logger functions.
+log(Level, Format, Args, _) ->
+	Function = case Level of
+		emergency -> error_msg;
+		alert -> error_msg;
+		critical -> error_msg;
+		error -> error_msg;
+		warning -> warning_msg;
+		notice -> warning_msg;
+		info -> info_msg;
+		debug -> info_msg
+	end,
+	error_logger:Function(Format, Args).