Add task-local context propagation

This commit introduces a new task-local context propagation mechanism to gRPC, enabling context to be preserved across await points in asynchronous tasks.

The implementation is based on a combination of:
*   **Otel FutureExt**: Adopts the extension trait pattern from OpenTelemetry's [[FutureExt](cci:2://file:///usr/local/google/home/sauravz/repos/tonic/grpc/src/context/extensions.rs:34:0-44:1)](https://docs.rs/opentelemetry/latest/opentelemetry/future/trait.FutureExt.html) to allow fluent context attachment via `.with_context(ctx)`.
*   **Tokio Task Local**: Implements a runtime-agnostic task-local storage mechanism similar to [`tokio::task_local!`](https://docs.rs/tokio/latest/tokio/macro.task_local.html), ensuring context is correctly scoped and propagated.

Changes:
*   Add `task_local_context` module for managing context scope.
*   Add FutureExt and StreamExt traits in `extensions.rs`.
*   Update `Context` to use `Arc<dyn Context>` for efficient sharing.

Author: sauravzg

grpc/src/context.rs

@@ -0,0 +1,7 @@
+mod context;
+mod extensions;
+mod task_local_context;
+
+pub use context::Context;
+pub use extensions::{FutureExt, StreamExt};
+pub use task_local_context::current;

grpc/src/context/context.rs

@@ -0,0 +1,119 @@
+use std::any::{Any, TypeId};
+use std::collections::HashMap;
+use std::sync::Arc;
+use std::time::Instant;
+
+/// A task-local context for propagating metadata, deadlines, and other request-scoped values.
+pub trait Context: Send + Sync + 'static {
+    /// Get the deadline for the current context.
+    fn deadline(&self) -> Option<Instant>;
+
+    /// Create a new context with the given deadline.
+    fn with_deadline(&self, deadline: Instant) -> Arc<dyn Context>;
+
+    /// Get a value from the context extensions.
+    fn get(&self, type_id: TypeId) -> Option<&(dyn Any + Send + Sync)>;
+
+    /// Create a new context with the given value.
+    fn with_value(&self, type_id: TypeId, value: Arc<dyn Any + Send + Sync>) -> Arc<dyn Context>;
+}
+
+#[derive(Clone, Default)]
+struct ContextInner {
+    deadline: Option<Instant>,
+    extensions: HashMap<TypeId, Arc<dyn Any + Send + Sync>>,
+}
+
+#[derive(Clone, Default)]
+pub(crate) struct ContextImpl {
+    inner: Arc<ContextInner>,
+}
+
+impl Context for ContextImpl {
+    fn deadline(&self) -> Option<Instant> {
+        self.inner.deadline
+    }
+
+    fn with_deadline(&self, deadline: Instant) -> Arc<dyn Context> {
+        let mut inner = (*self.inner).clone();
+        inner.deadline = Some(deadline);
+        Arc::new(Self {
+            inner: Arc::new(inner),
+        })
+    }
+
+    fn get(&self, type_id: TypeId) -> Option<&(dyn Any + Send + Sync)> {
+        self.inner.extensions.get(&type_id).map(|v| &**v as _)
+    }
+
+    fn with_value(&self, type_id: TypeId, value: Arc<dyn Any + Send + Sync>) -> Arc<dyn Context> {
+        let mut inner = (*self.inner).clone();
+        inner.extensions.insert(type_id, value);
+        Arc::new(Self {
+            inner: Arc::new(inner),
+        })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::time::Duration;
+
+    #[test]
+    fn default_context_has_no_deadline_or_extensions() {
+        let context = ContextImpl::default();
+        assert!(context.deadline().is_none());
+        assert!(context.get(TypeId::of::<i32>()).is_none());
+    }
+
+    #[test]
+    fn with_deadline_sets_deadline_and_preserves_original() {
+        let context = ContextImpl::default();
+        let deadline = Instant::now() + Duration::from_secs(5);
+        let context_with_deadline = context.with_deadline(deadline);
+
+        assert_eq!(context_with_deadline.deadline(), Some(deadline));
+        // Original context should remain unchanged
+        assert!(context.deadline().is_none());
+    }
+
+    #[test]
+    fn with_value_stores_extension_and_preserves_original() {
+        let context = ContextImpl::default();
+
+        #[derive(Debug, PartialEq)]
+        struct MyValue(i32);
+
+        let context_with_value = context.with_value(TypeId::of::<MyValue>(), Arc::new(MyValue(42)));
+
+        let value = context_with_value
+            .get(TypeId::of::<MyValue>())
+            .and_then(|v| v.downcast_ref::<MyValue>());
+        assert_eq!(value, Some(&MyValue(42)));
+
+        // Original context should not have the value
+        assert!(context.get(TypeId::of::<MyValue>()).is_none());
+    }
+
+    #[test]
+    fn with_value_overwrites_existing_extension_and_preserves_previous() {
+        let context = ContextImpl::default();
+
+        #[derive(Debug, PartialEq)]
+        struct MyValue(i32);
+
+        let ctx1 = context.with_value(TypeId::of::<MyValue>(), Arc::new(MyValue(10)));
+        let ctx2 = ctx1.with_value(TypeId::of::<MyValue>(), Arc::new(MyValue(20)));
+
+        let val1 = ctx1
+            .get(TypeId::of::<MyValue>())
+            .and_then(|v| v.downcast_ref::<MyValue>());
+        let val2 = ctx2
+            .get(TypeId::of::<MyValue>())
+            .and_then(|v| v.downcast_ref::<MyValue>());
+
+        assert_eq!(val1, Some(&MyValue(10)));
+        assert_eq!(val2, Some(&MyValue(20)));
+    }
+}

grpc/src/context/extensions.rs

@@ -0,0 +1,122 @@
+//! Extension traits for `Future` and `Stream` to provide context propagation.
+//!
+//! This module provides the [`FutureExt`] and [`StreamExt`] traits, which allow
+//! attaching a [`Context`] to a [`Future`] or [`Stream`]. This ensures that the
+//! context is set as the current task-local context whenever the future or stream
+//! is polled.
+
+use std::future::Future;
+use std::sync::Arc;
+use tokio_stream::Stream;
+
+use super::context::Context;
+use super::task_local_context;
+
+/// Extension trait for `Future` to provide context propagation.
+///
+/// This trait allows attaching a [`Context`] to a [`Future`], ensuring that the context
+/// is set as the current task-local context whenever the future is polled.
+///
+/// # Examples
+///
+/// ```rust
+/// # use std::sync::Arc;
+/// # use grpc::context::{Context, FutureExt};
+/// # async fn example() {
+/// let context = grpc::context::current();
+/// let future = async {
+///     // Context is available here
+///     assert!(grpc::context::current().deadline().is_none());
+/// };
+///
+/// future.with_context(context).await;
+/// # }
+/// ```
+pub trait FutureExt: Future {
+    /// Attach a context to this future.
+    ///
+    /// The context will be set as the current task-local context whenever the future is polled.
+    fn with_context(self, context: Arc<dyn Context>) -> impl Future<Output = Self::Output>
+    where
+        Self: Sized,
+    {
+        task_local_context::ContextScope::new(self, context)
+    }
+}
+
+impl<F: Future> FutureExt for F {}
+
+/// Extension trait for `Stream` to provide context propagation.
+///
+/// This trait allows attaching a [`Context`] to a [`Stream`], ensuring that the context
+/// is set as the current task-local context whenever the stream is polled.
+///
+/// # Examples
+///
+/// ```rust
+/// # use std::sync::Arc;
+/// # use grpc::context::{Context, StreamExt};
+/// # use tokio_stream::StreamExt as _;
+/// # async fn example() {
+/// let context = grpc::context::current();
+/// let stream = tokio_stream::iter(vec![1, 2, 3]);
+///
+/// let mut scoped_stream = stream.with_context(context);
+///
+/// while let Some(item) = scoped_stream.next().await {
+///     // Context is available here
+///     assert!(grpc::context::current().deadline().is_none());
+/// }
+/// # }
+/// ```
+pub trait StreamExt: Stream {
+    /// Attach a context to this stream.
+    ///
+    /// The context will be set as the current task-local context whenever the stream is polled.
+    fn with_context(self, context: Arc<dyn Context>) -> impl Stream<Item = Self::Item>
+    where
+        Self: Sized,
+    {
+        task_local_context::ContextScope::new(self, context)
+    }
+}
+
+impl<S: Stream> StreamExt for S {}
+
+#[cfg(test)]
+mod tests {
+    use super::super::context::ContextImpl;
+    use super::*;
+    use tokio_stream::StreamExt as _;
+
+    #[tokio::test]
+    async fn test_future_ext_attaches_context_correctly() {
+        let ctx = ContextImpl::default();
+        let deadline = std::time::Instant::now() + std::time::Duration::from_secs(10);
+        let ctx = ctx.with_deadline(deadline);
+
+        let future = async {
+            let current_ctx = super::task_local_context::current();
+            assert_eq!(current_ctx.deadline(), Some(deadline));
+        };
+
+        future.with_context(ctx).await;
+    }
+
+    #[tokio::test]
+    async fn test_stream_ext_attaches_context_correctly() {
+        let ctx = ContextImpl::default();
+        let deadline = std::time::Instant::now() + std::time::Duration::from_secs(10);
+        let ctx = ctx.with_deadline(deadline);
+
+        let stream = async_stream::stream! {
+            let current_ctx = super::task_local_context::current();
+            assert_eq!(current_ctx.deadline(), Some(deadline));
+            yield 1;
+        };
+
+        let scoped_stream = stream.with_context(ctx);
+        tokio::pin!(scoped_stream);
+        scoped_stream.next().await;
+    }
+}