What happened?

StateBackedIterable which is part of FnHarness is reporting size for every element without looking at cost of encoding. Other parts of SDK take into account isRegisterByteSizeObserverCheap and depending on response they sample.

To reproduce:

1. Implement ComposedCombine with 1k of accumulators
2. Create input of millions of elements
3. Combine globally.
4. Run on Dataflow v2

Straggler - "Operation ongoing" with following stacktrace fragment:

[..]
 at app//org.apache.beam.sdk.transforms.CombineFns$ComposedAccumulatorCoder.encode(CombineFns.java:649)
  at app//org.apache.beam.sdk.coders.Coder.getEncodedElementByteSize(Coder.java:297)
  at app//org.apache.beam.sdk.coders.LengthPrefixCoder.getEncodedElementByteSize(LengthPrefixCoder.java:109)
  at app//org.apache.beam.sdk.coders.Coder.registerByteSizeObserver(Coder.java:291)
  at app//org.apache.beam.fn.harness.state.StateBackedIterable$WrappedObservingIterator.next(StateBackedIterable.java:141)
  at app//org.apache.beam.sdk.transforms.CombineFns$ProjectionIterable$1.next(CombineFns.java:638)
  at app//com.google.common.collect.Iterators$5.computeNext(Iterators.java:672)
  at app//com.google.common.collect.AbstractIterator.tryToComputeNext(AbstractIterator.java:145)
  at app//com.google.common.collect.AbstractIterator.hasNext(AbstractIterator.java:140)
  at app//foo.bar.MyAccumulator.mergeAccumulators(MyAccumulator.java:418)
[..]

Expected outcome - no straggler.

Issue Priority

Priority: 2 (default / most bugs should be filed as P2)

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