core: impl Value for dyn Error + Send/Sync (#2066)

## Motivation

`Value` was already implemented for `dyn Error + 'static`, but rust
doesn't silently coerce trait objects. This means that passing an error
of type `dyn Error + Send + Sync + 'static` would not work. This is
related to #1308.

## Solution

Add impls for `dyn Error + …` variants for `Send`, `Sync`, and `Send +
Sync`. These extra impls just delegate to the existing `dyn Error +
'static` impl.

Also update one of the examples to use `dyn Error + Send + Sync` to
demonstrate that this works.

Refs: #1308

examples/examples/fmt/yak_shave.rs

@@ -7,7 +7,7 @@ use tracing::{debug, error, info, span, trace, warn, Level};
 // every time the instrumented function is called. The span is named after the
 // the function or method. Paramaters passed to the function are recorded as fields.
 #[tracing::instrument]
-pub fn shave(yak: usize) -> Result<(), Box<dyn Error + 'static>> {
+pub fn shave(yak: usize) -> Result<(), Box<dyn Error + Send + Sync + 'static>> {
     // this creates an event at the TRACE log level with two fields:
     // - `excitement`, with the key "excitement" and the value "yay!"
     // - `message`, with the key "message" and the value "hello! I'm gonna shave a yak."

tracing-core/src/field.rs

@@ -523,6 +523,39 @@ impl Value for dyn std::error::Error + 'static {
     }
 }
 
+#[cfg(feature = "std")]
+impl crate::sealed::Sealed for dyn std::error::Error + Send + 'static {}
+
+#[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+impl Value for dyn std::error::Error + Send + 'static {
+    fn record(&self, key: &Field, visitor: &mut dyn Visit) {
+        (self as &dyn std::error::Error).record(key, visitor)
+    }
+}
+
+#[cfg(feature = "std")]
+impl crate::sealed::Sealed for dyn std::error::Error + Sync + 'static {}
+
+#[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+impl Value for dyn std::error::Error + Sync + 'static {
+    fn record(&self, key: &Field, visitor: &mut dyn Visit) {
+        (self as &dyn std::error::Error).record(key, visitor)
+    }
+}
+
+#[cfg(feature = "std")]
+impl crate::sealed::Sealed for dyn std::error::Error + Send + Sync + 'static {}
+
+#[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+impl Value for dyn std::error::Error + Send + Sync + 'static {
+    fn record(&self, key: &Field, visitor: &mut dyn Visit) {
+        (self as &dyn std::error::Error).record(key, visitor)
+    }
+}
+
 impl<'a, T: ?Sized> crate::sealed::Sealed for &'a T where T: Value + crate::sealed::Sealed + 'a {}
 
 impl<'a, T: ?Sized> Value for &'a T