Changelog for switching periodreader default

Author: cijothomas

opentelemetry-sdk/CHANGELOG.md

@@ -24,6 +24,43 @@
      Migration Guidance: 
         - These methods were intended for log appenders. Keep the clone of the provider handle, instead of depending on above methods.
 
+- *Breaking* - `PeriodicReader` Updates
+
+  `PeriodicReader` no longer requires an async runtime by default. Instead, it now creates its own background thread for execution. This change allows metrics to be used in environments without async runtimes.
+
+  For users who prefer the previous behavior of relying on a specific `Runtime`, they can do so by enabling the feature flag **`experimental_metrics_periodicreader_with_async_runtime`**.
+
+  Migration Guide:
+
+1. *Default Implementation, requires no async runtime* (**Recommended**)
+   The new default implementation does not require a runtime argument. Replace the builder method accordingly:  
+   - *Before:* 
+     ```rust
+     let reader = PeriodicReader::builder(exporter, runtime::Tokio).build();
+     ```
+   - *After:*
+     ```rust
+     let reader = PeriodicReader::builder(exporter).build();
+     ```
+
+2. *Async Runtime Support*
+   If your application cannot spin up new threads or you prefer using async runtimes, update your code and enable the required feature flags.  
+
+   - *Before:*
+     ```rust
+     let reader = PeriodicReader::builder(exporter, runtime::Tokio).build();
+     ```
+   - *After:*
+     ```rust
+     let reader = PeriodicReader::builder(exporter)
+         .with_runtime(runtime::Tokio)
+         .build();
+     ```
+
+   *Requirements:*
+   - Enable the feature flag: `experimental_metrics_periodicreader_with_async_runtime`.  
+   - Enable one of the async runtime feature flags: `rt-tokio`, `rt-tokio-current-thread`, or `rt-async-std`. 
+
 ## 0.27.1
 
 Released 2024-Nov-27