mikemccand opened a new issue, #14993:
URL: https://github.com/apache/lucene/issues/14993
### Description
I was debugging an issue with `addIndexes`, turned on `IndexWriter`'s
`InfoStream` logging, and `ConcurrentMergeScheduler` periodically logs juicy
details like this:
```
MS 0 [2025-07-24T19:54:48.516458703Z; pool-4-thread-2]: updateMergeThreads
ioThrottle=false targetMBPerSec=20.0 MB/sec
merge thread Lucene Merge Thread #19 estSize=1137.2 MB (written=287.1 MB)
runTime=15.6s (stopped=0.0s, paused=0.0s) rate=unlimited
leave running at Infinity MB/sec
merge thread Lucene Merge Thread #20 estSize=0.0 MB (written=0.0 MB)
runTime=0.0s (stopped=0.0s, paused=0.0s) rate=unlimited
leave running at Infinity MB/sec
merge thread Lucene Merge Thread #21 estSize=0.0 MB (written=0.0 MB)
runTime=0.0s (stopped=0.0s, paused=0.0s) rate=unlimited
leave running at Infinity MB/sec
merge thread Lucene Merge Thread #22 estSize=0.0 MB (written=0.0 MB)
runTime=0.0s (stopped=0.0s, paused=0.0s) rate=unlimited
leave running at Infinity MB/sec
```
The top merge (`Lucene Merge Thread #19`) was a natural merge and it
correctly sets the `estSize` (`OneMerge.estimatedMergeBytes`),
[here](https://github.com/apache/lucene/blob/branch_10_2/lucene/core/src/java/org/apache/lucene/index/IndexWriter.java#L4896-L4906).
But the other merges all (incorrectly) show `estSize=0` because their merges
bypass `IndexWriter.registerMerge` and instead [register here]
(https://github.com/apache/lucene/blob/branch_10_2/lucene/core/src/java/org/apache/lucene/index/IndexWriter.java#L3295-L3299)
(in `AddIndexesMergeSource`).
This estimated size is useful for debugging ... e.g. [segment tracing
tools](https://githubsearch.mikemccandless.com/segments_15.html) use it. Also,
`ConcurrentMergeScheduler` uses this when throttles merges.
I think it should be a simple fix -- maybe factor out that code that
computes the `estimatedMergeSize` for natural merges today, so it can be shared
for merges triggered by `addIndexes` too?
### Version and environment details
_No response_
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