refactor: use split_last for multi-processor path, fix changelog

Use split_last() instead of index tracking for the multi-processor
on_end loop. Removes unnecessary break statement and makes the
clone-for-rest, move-for-last intent explicit.

Fix changelog entry to describe what actually changed without
overstating the scope clone savings.

Author: bryantbiggs

opentelemetry-sdk/CHANGELOG.md

@@ -2,7 +2,7 @@
 
 ## vNext
 
-- Store `InstrumentationScope` in `Arc` internally in `SdkTracer`, making tracer cloning cheaper and avoiding redundant scope clones in the multi-processor span export path.
+- Store `InstrumentationScope` in `Arc` internally in `SdkTracer`, making tracer clones cheaper (Arc refcount increment instead of deep copy). The multi-processor span export path now builds export data once instead of per processor.
 - Add 32-bit platform support by using `portable-atomic` for `AtomicI64` and `AtomicU64` in the metrics module. This enables compilation on 32-bit ARM targets (e.g., `armv5te-unknown-linux-gnueabi`, `armv7-unknown-linux-gnueabihf`).
 - `Aggregation` enum and `StreamBuilder::with_aggregation()` are now stable and no longer require the `spec_unstable_metrics_views` feature flag.
 - Fix `service.name` Resource attribute fallback to follow OpenTelemetry

opentelemetry-sdk/src/trace/span.rs

@@ -231,15 +231,11 @@ impl Span {
             processors => {
                 let export_data =
                     build_export_data(data, self.span_context.clone(), &self.tracer);
-                let last_idx = processors.len() - 1;
-                for (i, processor) in processors.iter().enumerate() {
-                    if i < last_idx {
-                        processor.on_end(export_data.clone());
-                    } else {
-                        processor.on_end(export_data);
-                        break;
-                    }
+                let (last, rest) = processors.split_last().unwrap();
+                for processor in rest {
+                    processor.on_end(export_data.clone());
                 }
+                last.on_end(export_data);
             }
         }
     }