sentry-tower: using the layer breaks with axum ~0.8 because it's not Sync

[syphar]

Environment

• sentry 0.35.0
• axum 0.8.1

Steps to Reproduce

When upgrading docs.rs itself to axum 0.8 I stumbled onto this issue.

use axum::{routing::get, Router};

#[tokio::main]
async fn main() {
    let app = Router::new()
        .route("/", get(|| async { "Hello, World!" }))
        .layer(sentry_tower::NewSentryLayer::new_from_top()); // this fails because the service is not `Sync` 

    let listener = tokio::net::TcpListener::bind("0.0.0.0:3000").await.unwrap();
    axum::serve(listener, app).await.unwrap();
}

With axum 0.8 all Layers & services are expected to be Sync (tokio-rs/axum#2473).

sentry_tower::SentryLayer and ::SentryService are only Sync if the given Request is Sync, which the default axum::request::Request isn't.

So my guess is that every axum user will run into this issue.

Expected Result

Since the Request is only used in a function argument in the HubProvider, I assume that the service / layer could totally be Sync.

Actual Result

error[E0277]: `(dyn HttpBody<Data = axum::body::Bytes, Error = axum::Error> + Send + 'static)` cannot be shared between threads safely
    --> src/main.rs:8:16
     |
8    |         .layer(sentry_tower::NewSentryLayer::new_from_top());
     |          ----- ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `(dyn HttpBody<Data = axum::body::Bytes, Error = axum::Error> + Send + 'static)` cannot be shared between thre
ads safely
     |          |
     |          required by a bound introduced by this call
     |

potential solution?

From what I see, the solution could be something like this. It definitely compiles in docs.rs. I'm not sure about what tests would be expected.