health_check for dynamic upstream (1 reply)

Hi!
I faced with trouble.
I have to upstreams and I route requests to them by some headers like $user_agent or $http_x_forwarded_proto
so it looks like:

map $some_header $backend {
default "first";
some_value "second";
}
upstream first {}
upstream second {}

server {
proxy_pass $backend;
health_check;
}

I get error.
So my question how I can use health_check for this case?
Thank you for any help.
I can cookies and I 'd love share it :P