add changelog fragment for unordered stateless transforms and scheduling pressure metric

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

Author: ArunPiduguDD

changelog.d/25070_unordered_stateless_transforms.enhancement.md

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+Added a new global configuration option `preserve_ordering_stateless_transforms` (default: `true`) that
+controls whether stateless transforms maintain event ordering when processing concurrently. Setting this
+to `false` allows concurrent tasks to complete out of order, which can improve throughput in CPU-bound
+transform workloads at the cost of ordering guarantees.
+
+Added a new internal metric `estimated_concurrent_transform_scheduling_pressure` that measures the
+fraction of in-flight concurrent transform tasks that completed while a new input batch was being
+waited on. A value near 1.0 indicates the transform is CPU-bound and may benefit from setting
+`preserve_ordering_stateless_transforms: false`.
+
+authors: ArunPiduguDD