kubectl completion fish - label support required

**What would you like to be added**:

When running fish shell, currently completions do not support label support yet.

For example:

`kubectl get pods -l app=` # tab tab

I need a feature, where you can press tab tab when doing for example get pods or other commands and label suggestions are automatically completed. I don't know if this is already implemented.

Running

`kubectl completion fish` as far as I understand does not yet support it.

Can it be implemented?

**Why is this needed**:

Comfortable function, time saving.

Start by running `kubectl completion fish` and tracing how fish completions handle `kubectl get pods -l app=`. Compare the generated completion behavior for label values across get and other commands; done means pressing Tab offers label suggestions in fish where label selectors are accepted.

Written by the indexing model from the issue text.