Support partial reads of array-valued signals across backends?

[PierreSchnizer]

We are seeing performance issues when working with large array-valued signals (e.g. EPICS waveforms, TANGO spectrum attributes), where only a small subset of the data is typically needed.

In EPICS, this can be addressed using the n_elements feature to read only the first N elements of a waveform.
A minimal implementation to illustrate the idea is available at #1253

The PR is not intended as a final design, but rather to provide a concrete example to illustrate the idea and make the discussion more tangible.

In practice, this kind of optimization can have a significant impact. For example, at BESSY II, reading out all beam position monitors of the accelerator currently doubles measurement time due to the volume of data transferred. Users are forced to limit the number of devices instead of using all available data sources.

This raises a more general question:

• Should ophyd-async support partial reads of array-valued signals across backends?
• If so, what would a backend-agnostic API look like?
  *Should this be:
  • a property of the signal definition?
  • a per-read option?
  • something else?

I can provide more detailed examples and use cases. It would be helpful to understand what kind of examples would be most useful to guide the discussion.