Manage opentracing with nginx-variable

[mpiexec]

Is it possible to use nginx-variable to manage opentracing ability in some conditions, ie:

map $arg_debug $trace {
    ~. on;
    default off;
}

server {
    # ...skipped...
    location = /login {
        opentracing $trace;
        proxy_pass http://backend;
    }
}

Now I've got that error message:

nginx: [emerg] invalid value "$trace" in "opentracing" directive, it must be "on" or "off"

[miry]

I think you should create a location with regexp instead of variables to make it work. I try to find the person whom would contribute to the project.