Redo OTLPMetricExporter unit tests of max_export_batch_size to use real export (#5036)

* Full replace of test_split_metrics

* Add exported contents assertions

* Changelog

Author: tammy-baylis-swi

CHANGELOG.md

@@ -59,6 +59,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   ([#5015](https://github.com/open-telemetry/opentelemetry-python/pull/5015))
 - `opentelemetry-sdk`: cache TracerConfig into the tracer, this changes an internal interface. Only one Tracer with the same instrumentation scope will be created
   ([#5007](https://github.com/open-telemetry/opentelemetry-python/pull/5007))
+- Redo OTLPMetricExporter unit tests of `max_export_batch_size` to use real `export`
+  ([#5036](https://github.com/open-telemetry/opentelemetry-python/pull/5036))
 
 ## Version 1.40.0/0.61b0 (2026-03-04)