Changes to Get available metrics' definitions

the items/items/category response's property type changed from no type to string for status 200

added the non-success response with the status 400

added the non-success response with the status 401

added the non-success response with the status 403

added the non-success response with the status 404

added the non-success response with the status 429

the response property items became required for the status 200

the endpoint scheme security client_credentials was added to the API

endpoint added