blocking: #[track_caller] for spawn_blocking (#4616)

Annotates spawn_blocking() with #[track_caller] in order to produce the
correct tracing span spawn locations that show the caller as the spawn
point.

tokio/src/runtime/blocking/pool.rs

@@ -104,6 +104,7 @@ const KEEP_ALIVE: Duration = Duration::from_secs(10);
 /// Runs the provided function on an executor dedicated to blocking operations.
 /// Tasks will be scheduled as non-mandatory, meaning they may not get executed
 /// in case of runtime shutdown.
+#[track_caller]
 pub(crate) fn spawn_blocking<F, R>(func: F) -> JoinHandle<R>
 where
     F: FnOnce() -> R + Send + 'static,