Adding metadata/labels to a route? about cowboy

[rlipscombe]

I've been wracking my brains for ways to wrap cowboy_router:compile or the cowboy_router middleware, or to wrap the individual handlers, but I can't come up with anything that doesn't feel like a hack.

[rlipscombe]

After some thought, I've come up with the following, which doesn't require any cowboy changes:

• Replace the current Handler with a tuple, {Handler, HandlerOpts}
• Replace the current HandlerOpts with HandlerMetadata.

Add a middleware that goes in between cowboy_router and cowboy_handler that does this:

execute(Req, Env = #{handler := {Handler, HandlerOpts}, handler_opts := Metadata}) ->
    Env2 = Env#{handler => Handler, handler_opts => HandlerOpts, handler_metadata => Metadata},
    {ok, Req, Env2}.

Getting handler_metadata into user_data where the metrics callback can see it is left as an exercise for the reader.