core: impl field::Value for Option<T> (tokio-rs#1585)

## Motivation

Building a span with some field might be `None` is bother now.

Before:
```rust
let span = info!("example", data = Empty );
if let Some(data) = foo() {
    span.record("data", &data);
}
```

After:
```rust
let span = info!("example", data = foo() );
```

Co-authored-by: DCjanus <[email protected]>
Co-authored-by: Eliza Weisman <[email protected]>

tracing-core/src/field.rs

@@ -643,7 +643,7 @@ impl<T: fmt::Display> fmt::Display for DisplayValue<T> {
 
 impl<T: fmt::Debug> crate::sealed::Sealed for DebugValue<T> {}
 
-impl<T: fmt::Debug> Value for DebugValue<T>
+impl<T> Value for DebugValue<T>
 where
     T: fmt::Debug,
 {
@@ -664,6 +664,16 @@ impl Value for Empty {
     fn record(&self, _: &Field, _: &mut dyn Visit) {}
 }
 
+impl<T: Value> crate::sealed::Sealed for Option<T> {}
+
+impl<T: Value> Value for Option<T> {
+    fn record(&self, key: &Field, visitor: &mut dyn Visit) {
+        if let Some(v) = &self {
+            v.record(key, visitor)
+        }
+    }
+}
+
 // ===== impl Field =====
 
 impl Field {