Count otel.sdk.processor.{span,log}.processed at exporter-submit time (#5472)

* Count processor.processed at exporter-submit time, not export completion

* Add changelog fragment

* Trim comments and fix changelog wording

* Clarify changelog covers simple and batch processors

* Use single-line changelog fragment without redundant PR link

Assisted-by: Claude Opus 4.8

* Add test that shutdown-dropped records are not counted as processed

Assisted-by: Claude Opus 4.8

* Move SimpleSpanProcessor finish_items outside the export try/except

Assisted-by: Claude Opus 4.8

* Simplify shutdown metrics test to call on_emit directly

Assisted-by: Claude Opus 4.8