use single threaded types for OTel metrics

Author: martintmk

opentelemetry-sdk/CHANGELOG.md

@@ -3,6 +3,8 @@
 ## vNext
 
 - *Breaking* Make `force_flush()` in `PushMetricExporter` synchronous
+- Breaking: The `Runtime` trait has been simplified and refined. See the [#2641](https://github.com/open-telemetry/opentelemetry-rust/pull/2641)
+  for the changes.
 
 ## 0.28.0