stankiewicz opened a new pull request, #38230:
URL: https://github.com/apache/beam/pull/38230
## Description
This PR refactors the metadata propagation logic in `ReduceFnRunner` to
support extensible metadata.
Previously, metadata tracking (specifically `causedByDrain`) was not stored
in state at all during execution in `ReduceFnRunner`, which caused metatada
loss failures when firing timers or merging panes. This PR fixes that by
introducing a unified state map for element metadata.
To make it easier to add future payloads (such as OpenTelemetry context maps
or CDC insert/update markers) without modifying method signatures, this change
groups targeted fields into a unified container and offloads combination rules
to a clean aggregator class.
### Key Changes
- **Runner Fix**: Added stateful tracking using `METADATA_TAG` (persisting
`PipelineMetadata`) in `ReduceFnRunner`, ensuring metadata is no longer lost
during grouping.
- **Extensible Container**: Created `PipelineMetadata` (guided by
`@AutoValue`) that groups element metadata together.
- **Metadata Combiners**: Encapsulated combination logic within
`PipelineMetadataCombiner`.
------------------------
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