Is your feature request related to a problem? Please describe.
Currently, apollo supports the grayscale feature for properties namespaces.

However, it's not available for non-properties namespaces, e.g. xml/yml, which is strongly required by many users, see #2927.

Describe the solution you'd like
Support grayscale feature for non-properties namespaces, to make it easier, we may only support grayscale the whole text instead of some keys.