Should real-time measurement take place at the connection point or by using sub-metering?

Either one works. What is important is that the requirements for measurement are met, both during testing and operation, and that provision of the ancillary service can be verified.

Measuring at the connection point may make it more difficult to verify provision, as this must take into account other consumption or production.

For this reason, it may be better to measure closer to the resource itself. In the case of sub-metering, it is a prerequisite that the system being measured is isolated, i.e. that other processes do not affect the sub-metering.