Is grpc keepalive supported ?

[bcoz123]

Hello everyone,
In the latest version(1.13.10)
Does ‘grpc_pass’ support the ‘keepalive’ option ?
My configuration is:

...
http {
    ...
    upstream backend {
        server localhost:50051;
        keepalive 300;
    }
    server {
        listen 80 http2;
        location / {
            grpc_pass grpc://backend;
       }
    ...
}
...

It seems not work,
Because I can still see for every request from the front, 
Nginx create a new tcp connection between nginx and backend every time.
Is there something wrong in my configuration file?
Or just it is not supported in this version ?

Thanks

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,279261,279261#msg-279261