Question Pagespeed - shared memory metadata cache (shm)

[Servus]

Hi,
I've read the whole pagespeed documentation over a hundred times and some points I do not understand, or I don't know how to realize them.
I tested pagespeed in various configurations with redis, memcached and tmpfs. Only one time the /rname/ directory was created, I think it was together with redis.

I'm not able to create the shm cache properly. Would be great if someone could help me to configure it correctly.
Where and how do I have to place this? (I put it in /etc/nginx/nginx.conf, but I can't put the vhost part there):

pagespeed CreateSharedMemoryMetadataCache "/var/cache/pagespeed/" 51200;
server {
  listen 80;
  server_name www.example.com;
  pagespeed FileCachePath "/var/cache/pagespeed/";
}
server {
  listen 80;
  server_name alt.example.com;
  pagespeed FileCachePath "/var/cache/pagespeed/";
}

The first part

pagespeed CreateSharedMemoryMetadataCache "/var/cache/pagespeed/" 51200;

I can only place in /etc/nginx/nginx.conf (in additional nginx directives it's not possible).
How and where shall I place the rest? Do I have to create a custom vhost config? Please, could you tell me how if so?

Kindest regards