This version expects simple space separated keyword_list param in tms md.
Otherwise we could use standard keyword_list with vocabulary simplified as space separated list with following snippet in tms_capabilities.xml

{{if service.keyword_list and len(service.keyword_list) > 0}}
{{for list in service.keyword_list}}{{py: kw=bunch(default='', **list)}}{{" ".join(map(lambda k : '"'+k+'"', kw.keywords))}}{{endfor}}
{{endif}}

Thanks for this work!

Can you please provide an example configuration with all provided keywords and add it to the documentation?

It should work as similar as possible to the wms metadata described here: https://mapproxy.github.io/mapproxy/latest/services.html#full-example

Could you also provide a very simple test here:

You only need to provide one metadata key and check for it's appearance in the result, I think that's enough to ensure the basic functionality of this feature.

Added simple test and full example in doc for second approach with standard keyword_list (we would prefer). Should we notice in doc, that if no special service tms md block is provided the wms md entries are used by default?

Yes that would be useful. It should go into a md section, just like for the use_grid_names option