Lager is a popular logging framework for Erlang applications. To get it working with Cowboy (assuming you’re using erlang.mk), you need to add it as a dependency to your Makefile:

DEPS = cowboy lager

and then add the parse transform to the options for erlc (as shown here):

include erlang.mk
ERLC_OPTS += +'{parse_transform, lager_transform}'

(alternatively, you could add a compile header to each file that needs it). You also need to add lager to your app.src file:

    {applications, [
        kernel,
        stdlib,
        cowboy,
        lager
    ]},
]}.

so that it is started as part of your release. You can find an example repo here.