## Summary
Adds two counters to the native realtime backend so we can see how much
duplicate row serialization the change router does per batch. When a run
changes it can match several held feeds at once (a run subscription plus
one or more tag/list feeds), and today each matching feed serializes
that run's wire value independently. These counters quantify that
fan-out so we can decide whether a shared serialization step is worth
it.
## What they measure
- `realtime_native.emission_run_serializations`: total wire-value
serializations performed across feeds per batch (what the current path
does).
- `realtime_native.emission_distinct_serializations`: distinct (columns,
run) rows those serializations cover (what a serialize-once-per-batch
step would do).
Average feeds-per-run is `run_serializations / distinct_serializations`,
and `1 - distinct / run_serializations` is the serialization work a
shared step could save. Wired through a new optional `onEmissionFanout`
callback on the router. No behavior change.
Eric Allam
a2d382b2be