Use consistent element count for progress counter.

This influences how the SDK communicates progress and how splitting is performed.

We currently inspect the element count of the output (which has inconsistent definitions across SDKs, see BEAM-9934). Instead, we can move to using an explicit, separate metric.

This can currently lead to incorrect progress reporting and in some cases even a crash in the UW depending on the SDK, and makes it more difficult (but not impossible) to fix element counts in the future.