[#29180][prism] Return total element count to progress loop. Split less aggressively.

[lostluck]

Return the total element count to the progress loop, and use as part of the split computation.

This avoids prism being hyper aggressive at splitting SDFs, and but still splits a bundle when stuck.

While out of scope for this PR, the total element count will monotonically increase during the bundle's lifetime, and will indicate if any progress is being made in the bundle, not simply at the root transform. (Within Beam's domain of course, since we don't have a way to measure external progress.) eg. We could track the progress at the last few ticks and only split if progress dips below our estimated progress.

Fixes #29180

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[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (6066af3) 38.23% compared to head (86cd617) 38.23%.
Report is 13 commits behind head on master.

Files	Patch %	Lines
...beam/runners/prism/internal/jobservices/metrics.go	75.00%	3 Missing ⚠️
...pkg/beam/runners/prism/internal/jobservices/job.go	0.00%	1 Missing ⚠️
sdks/go/pkg/beam/runners/prism/internal/stage.go	90.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #29968   +/-   ##
=======================================
  Coverage   38.23%   38.23%           
=======================================
  Files         696      696           
  Lines      101879   101889   +10     
=======================================
+ Hits        38953    38961    +8     
- Misses      61309    61311    +2     
  Partials     1617     1617           

Flag	Coverage Δ
go	53.94% <78.26%> (+<0.01%)	⬆️

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[riteshghorse]

LGTM, thank you!

[lostluck]

Thanks for the review!