Add computed field only to required content types

Problem/Motivation

If you use for example "RESTful Web Services" core module to get nodes. Then for all content types that are not specified in the configuration, you will get a redundant field: "node_read_time":[{"value":null}]

Proposed resolution

I suggest checking the configuration (for which content types is activated) not inside Computed field, but at the stage of adding Computed field. In order not to add a field to those content types that are not needed.