[tracing-subscriber]: add chrono crate implementations of FormatTime (t…

…okio-rs#2690)

Issue tokio-rs#2080 explains that it's not
possible to soundly use
[`tracing_subscriber::fmt::time::LocalTime`](https://docs.rs/tracing-subscriber/latest/tracing_subscriber/fmt/time/struct.LocalTime.html)
in a multithreaded context. It proposes adding alternative time formatters that
use the [chrono crate](https://docs.rs/chrono/latest/chrono/) to workaround
which is what this PR offers.

A new source file 'chrono_crate.rs' is added to the 'tracing-subscriber'
package implementing `mod chrono_crate` providing two new tag types `LocalTime`
and `Utc` with associated `time::FormatTime` trait implementations that call
`chrono::Local::now().to_rfc3339()` and `chrono::Utc::now().to_rfc3339()`
respectively. Simple unit-tests of the new functionality accompany the
additions.
---------

Co-authored-by: David Barsky <[email protected]>
Co-authored-by: Shayne Fletcher <[email protected]>

tracing-subscriber/src/fmt/time/mod.rs

@@ -28,6 +28,18 @@ pub use chrono_crate::ChronoLocal;
 #[cfg_attr(docsrs, doc(cfg(feature = "chrono")))]
 pub use chrono_crate::ChronoUtc;
 
+/// [`chrono`]-based implementation for [`FormatTime`].
+#[cfg(feature = "chrono")]
+mod chrono_crate;
+
+#[cfg(feature = "chrono")]
+#[cfg_attr(docsrs, doc(cfg(feature = "chrono")))]
+pub use chrono_crate::ChronoLocal;
+
+#[cfg(feature = "chrono")]
+#[cfg_attr(docsrs, doc(cfg(feature = "chrono")))]
+pub use chrono_crate::ChronoUtc;
+
 /// A type that can measure and format the current time.
 ///
 /// This trait is used by `Format` to include a timestamp with each `Event` when it is logged.