Support to define the subscriptions in separate configuration file.

[amuntean-godaddy]

I have use case where the subscriptions are generated by a different tool and then needs to merged with gnmi config.

It would be great if support for multiple configuration file can be added.

Example:

1. Using globing gnmic subscribe --config /app/gnmi-*
2. Using same flag multiple time: gnmic subscribe --config /app/config --config /app/subs-config

[karimra]

Merging YAML files can easily be done with tools like yq: https://mikefarah.gitbook.io/yq/operators/reduce#merge-all-yaml-files-together

I think this would be an unnecessary complication for gNMIc.