Fix minor issues in some examples (#806)

* remove unused `axum`'s dependency:`tokio-util`

* fix `examples/todos`'s `async fn todos_index` iter_overeager_cloned

* Add docs to `/examples/async-graphql`, just like other xamples.

* remove `examples/async-graphql` unused dependencies `tracing-subscriber` and `trace`

* `examples/chat` deps `trace` and `tracing-subscriber` never be used. Add trace `debug` to `chat`

* remove `examples/print-request-response` unused dependency `axum-extra`

* remove `examples/prometheus-metrics` unused dependency `axum-extra`

* remove `examples/reverse-proxy` unused dependencies `tracing-subscriber` and `trace`

* `examples/chat` fmt fix

examples/async-graphql/src/main.rs

@@ -1,3 +1,11 @@
+//! Example async-graphql application.
+//!
+//! Run with
+//!
+//! ```not_rust
+//! cargo run -p example-async-graphql
+//! ```
+
 mod starwars;
 
 use async_graphql::{