docs: add CHANGELOG entry for SpanProcessor::on_end breaking change

Author: bryantbiggs

opentelemetry-sdk/CHANGELOG.md

@@ -25,6 +25,14 @@
 - Fix ObservableCounter and ObservableUpDownCounter now correctly report only data points from the current measurement cycle, removing stale attribute combinations that are no longer observed. [#3248][3248]
 - Fix panic when `SpanProcessor::on_end` calls `Context::current()` ([#3262][3262]).
   - Updated `SpanProcessor::on_end` documentation to clarify that `Context::current()` returns the parent context, not the span's context
+- **Breaking** `SpanProcessor::on_end` now takes `&mut FinishedSpan` instead of `SpanData`.
+  Processors that only need to read span data can use the `ReadableSpan` trait without cloning.
+  Processors that need ownership call `FinishedSpan::consume()` — the last processor receives
+  ownership via move (zero-copy), earlier processors receive a clone.
+  Supersedes [#2962](https://github.com/open-telemetry/opentelemetry-rust/pull/2962).
+  Relates to [#2940](https://github.com/open-telemetry/opentelemetry-rust/issues/2940),
+  [#2726](https://github.com/open-telemetry/opentelemetry-rust/issues/2726),
+  [#2939](https://github.com/open-telemetry/opentelemetry-rust/issues/2939).
 
 [3227]: https://github.com/open-telemetry/opentelemetry-rust/pull/3227
 [3277]: https://github.com/open-telemetry/opentelemetry-rust/pull/3277