attributes: ensure res and err events inherit target (tokio-rs#…

…2184)

## Motivation

Currently, when an `#[instrument]` attribute has an overridden target,
the events generated by `ret` and `err` arguments do not inherit that
target.

For example, if I write

```rust
#[tracing::instrument(target = "some_target", ret)
fn do_stuff() -> Something {
    // ...
}
```

the `do_stuff` span will have the target "some_target", but the return
value event generated by `ret` will have the current module path as its
target, and there is no way to change the return value event's target.

## Solution

This branch changes the macro expansion for `#[instrument]` with the
`ret` and/or `err` arguments so that an overridden target is propagated
to the events generated by `ret` and `err`.

Fixes tokio-rs#2183

tracing-attributes/tests/ret.rs

@@ -66,6 +66,33 @@ fn test_custom_target() {
     handle.assert_finished();
 }
 
+#[test]
+fn test_custom_target() {
+    let filter: EnvFilter = "my_target=info".parse().expect("filter should parse");
+    let span = span::mock()
+        .named("ret_with_target")
+        .with_target("my_target");
+
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(span.clone())
+        .enter(span.clone())
+        .event(
+            event::mock()
+                .with_fields(field::mock("return").with_value(&tracing::field::debug(42)))
+                .at_level(Level::INFO)
+                .with_target("my_target"),
+        )
+        .exit(span.clone())
+        .drop_span(span)
+        .done()
+        .run_with_handle();
+
+    let subscriber = subscriber.with(filter);
+
+    with_default(subscriber, ret_with_target);
+    handle.assert_finished();
+}
+
 #[instrument(level = "warn", ret)]
 fn ret_warn() -> i32 {
     42