chore: Add release notes for 0.30

docs/release_0.30.md

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+# Release Notes 0.30
+
+OpenTelemetry Rust 0.30 introduces a few breaking changes to the
+`opentelemetry_sdk` crate in the `metrics` feature. These changes were essential
+to drive the Metrics SDK towards stability. With this release, the Metrics SDK
+is officially declared stable. The Metrics API was declared stable last year,
+and previously, the Logs API, SDK, and OTel-Appender-Tracing were also marked
+stable. Importantly, no breaking changes have been introduced to components
+already marked as stable.
+
+It is worth noting that the `opentelemetry-otlp` crate remains in a
+Release-Candidate state and is not yet considered stable. With the API and SDK
+for Logs and Metrics now stable, the focus will shift towards further refining
+and stabilizing the OTLP Exporters in upcoming releases. Additionally,
+Distributed Tracing is expected to progress towards stability, addressing key
+interoperability challenges.
+
+For detailed changelogs of individual crates, please refer to their respective
+changelog files. This document serves as a summary of the main changes.
+
+## Key Changes
+
+### Metrics SDK Improvements
+
+1. **Stabilized "view" features**: Previously under an experimental feature
+   flag, views can now be used to modify the name, unit, description, and
+   cardinality limit of a metric. Advanced view capabilities, such as changing
+   aggregation or dropping attributes, remain under the experimental feature
+   flag.
+
+2. **Cardinality capping**: Introduced the ability to cap cardinality and
+   configure limits using views.
+
+3. **Polished public API**: Refined the public API to hide implementation
+   details from exporters, enabling future internal optimizations and ensuring
+   consistency. Some APIs related to authoring custom metric readers have been
+   moved behind experimental feature flags. These advanced use cases require
+   more time to finalize the API surface before being included in the stable
+   release.
+
+### Context-Based Suppression
+
+Added the ability to suppress telemetry based on Context. This feature prevents
+telemetry-induced-telemetry scenarios and addresses a long-standing issue. Note
+that suppression relies on proper context propagation. Certain libraries used in
+OTLP Exporters utilize `tracing` but do not adopt OpenTelemetry's context
+propagation. As a result, not all telemetry is automatically suppressed with
+this feature. Improvements in this area are expected in future releases.
+
+## Next Release
+
+In the [next
+release](https://github.com/open-telemetry/opentelemetry-rust/milestone/22), the
+focus will shift to OTLP Exporters and Distributed Tracing, specifically
+resolving
+[interoperability](https://github.com/open-telemetry/opentelemetry-rust/issues/2420)
+issues with `tokio-tracing` and other fixes required to drive Distributed
+Tracing towards stability.
+
+## Acknowledgments
+
+Thank you to everyone who contributed to this milestone. We welcome your
+feedback through GitHub issues or discussions in the OTel-Rust Slack channel
+[here](https://cloud-native.slack.com/archives/C03GDP0H023).
+
+We are also excited to announce that [Anton Grübel](https://github.com/gruebel)
+and [Björn Antonsson](https://github.com/bantonsson) have joined the OTel Rust
+project as Approvers.