meta: Add missing sentry-tower README

Author: Swatinem

sentry-tower/README.md

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+<p align="center">
+    <a href="https://sentry.io" target="_blank" align="center">
+        <img src="https://sentry-brand.storage.googleapis.com/sentry-logo-black.png" width="280">
+    </a>
+</p>
+
+# Sentry Rust SDK: sentry-tower
+
+Adds support for automatic hub binding for each request received by the Tower server (or client,
+though usefulness is limited in this case).
+
+This allows breadcrumbs collected during the request handling to land in a specific hub, and
+avoid having them mixed across requests should a new hub be bound at each request.
+
+## Examples
+
+```rust
+use sentry_tower::NewSentryLayer;
+
+// Compose a Tower service where each request gets its own Sentry hub
+let service = ServiceBuilder::new()
+    .layer(NewSentryLayer::<Request>::new_from_top())
+    .timeout(Duration::from_secs(30))
+    .service(tower::service_fn(|req: Request| format!("hello {}", req)));
+```
+
+More customization can be achieved through the `new` function, such as passing a [`Hub`]
+directly.
+
+```rust
+use sentry::Hub;
+use sentry_tower::SentryLayer;
+
+// Create a hub dedicated to web requests
+let hub = Arc::new(Hub::with(|hub| Hub::new_from_top(hub)));
+
+// Compose a Tower service
+let service = ServiceBuilder::new()
+    .layer(SentryLayer::<_, _, Request>::new(hub))
+    .timeout(Duration::from_secs(30))
+    .service(tower::service_fn(|req: Request| format!("hello {}", req)));
+```
+
+The layer can also accept a closure to return a hub depending on the incoming request.
+
+```rust
+use sentry::Hub;
+use sentry_tower::SentryLayer;
+
+// Compose a Tower service
+let hello = Arc::new(Hub::with(|hub| Hub::new_from_top(hub)));
+let other = Arc::new(Hub::with(|hub| Hub::new_from_top(hub)));
+
+let service = ServiceBuilder::new()
+    .layer(SentryLayer::new(|req: &Request| match req.as_str() {
+        "hello" => hello.clone(),
+        _ => other.clone(),
+    }))
+    .timeout(Duration::from_secs(30))
+    .service(tower::service_fn(|req: Request| format!("{} world", req)));
+```
+
+When using Tonic, the layer can be used directly by the Tonic stack:
+
+```rust
+use sentry_tower::NewSentryLayer;
+use hello_world::{*, greeter_server::*};
+
+struct GreeterService;
+
+#[tonic::async_trait]
+impl Greeter for GreeterService {
+    async fn say_hello(&self, req: Request<HelloRequest>) -> Result<Response<HelloReply>, Status> {
+        let HelloRequest { name } = req.into_inner();
+        if name == "world" {
+            capture_anyhow(&anyhow!("Trying to greet a planet"));
+            return Err(Status::invalid_argument("Cannot greet a planet"));
+        }
+        Ok(Response::new(HelloReply { message: format!("Hello {}", name) }))
+    }
+}
+
+Server::builder()
+    .layer(NewSentryLayer::new_from_top())
+    .add_service(GreeterServer::new(GreeterService))
+    .serve("127.0.0.1:50051".parse().unwrap())
+    .await?;
+```
+
+## Resources
+
+License: Apache-2.0
+
+- [Discord](https://discord.gg/ez5KZN7) server for project discussions.
+- Follow [@getsentry](https://twitter.com/getsentry) on Twitter for updates