Studio fields are prefixed by x_studio, not only x_ which are simple custom
fields. This commit update the field name check to correctly include fields
created through studio.

As studio automatically add x_studio as prefix help is updated to tell
users to only use kpi_ as prefix for their digest fields. Result will
be x_studio_kpi_foo and x_studio_kpi_foo_value which are the value digest
needs to run.

This commit is linked to task ID 1887619.