Disable add_kubernetes_metadata if no matchers found

[ChrsMark]

Fixes #13705

As far as I can see, the problem with Beats other than Filebeat and Metricbeat is that they do not have default matchers. In the docs is mentioned that Filebeat enables logs_path matcher by default (Metricbeat, enables field matcher).

I'm not sure if we should add default matchers for all other Beats.
I think that ignoring that error and just disable the processor should be ok.
@jsoriano how it sounds to you?

cc: @andrewkroh

[jsoriano]

It looks good to me, I think that in any case we have to do this after #13473. I don't think we should add default matchers to all beats, it may not make any sense in some of them.

I was wondering why you moved the code from before the checks for the client configuration (and not only replacing the current error), but it is probably ok because users can define their own indexers and matchers in the configuration.

I think that in any case we could also remove add_kubernetes_metadata from the configuration files of beats without default indexers and matchers (for example with conditions as done here).

Also we have to take into account that we are removing the kubernetes and docker code from builds of platforms without docker support (see #13509), this will make all beats to badly fail on startup with default configuration on these platforms, as these processors won't exist. Not sure what to do about this, maybe we should reconsider #13068.

[ChrsMark]

Jaime thank you for commenting on this.

I think this is reviewable now and we can handle extra concerns in follow-ups as I proposed on #13705.

[ChrsMark]

Thanks for helping with this guys!