This Google Docs Add on enable us to add Custom File Properties to a file. You can also use Drive API to add your own properties to a Google Drive file. These properties are stored as key/value pairs on the file.

I did not find a way using which I can add custom key-value properties using a UI. So I build this.

You can read more about custom properties

Use Case:

I want Drive api to fetch only those docs which I have published. In this scenario I will add custom properties to those docs eg {type:published} then using Drive api I will fetch only such docs. As Properties can also be used in search expressions.

After installing, you can access this from Add-ons ( beside the tools menu). On Clicking Add metadata a sidebar will open on the right hand side.You can manage the custom properties ( CRUD ) from that side bar.

Note: This is an open source item. You can view the code @ https://github.com/imox2/gdoc-metadata-adder . 

p.s : If you want to make a similar add on for example for google sheet, it will take minor effort to change the code and get it done