Add json-subscriber to ecosystem (#3121)

## Motivation

I've created a library for better customization of JSON log lines and would like to make it more discoverable.

This subscriber could help with a lot of issues such as #1531 

## Solution

Add `json-subscriber` to the ecosystem.

tracing/README.md

@@ -379,6 +379,7 @@ maintained by the `tokio` project. These include:
 - [`sentry-tracing`] provides a layer for reporting events and traces to [Sentry].
 - [`tracing-loki`] provides a layer for shipping logs to [Grafana Loki].
 - [`tracing-logfmt`] provides a layer that formats events and spans into the logfmt format.
+- [`json-subscriber`] provides a layer for emitting JSON logs. The output can be customized much more than with [`FmtSubscriber`]'s JSON output.
 
 If you're the maintainer of a `tracing` ecosystem crate not listed above,
 please let us know! We'd love to add your project to the list!
@@ -410,6 +411,7 @@ please let us know! We'd love to add your project to the list!
 [`tracing-loki`]: https://crates.io/crates/tracing-loki
 [Grafana Loki]: https://grafana.com/oss/loki/
 [`tracing-logfmt`]: https://crates.io/crates/tracing-logfmt
+[`json-subscriber`]: https://crates.io/crates/json-subscriber
 
 **Note:** that some of the ecosystem crates are currently unreleased and
 undergoing active development. They may be less stable than `tracing` and

tracing/src/lib.rs

@@ -817,6 +817,7 @@
 //!  - [`reqwest-tracing`] provides a middleware to trace [`reqwest`] HTTP requests.
 //!  - [`tracing-cloudwatch`] provides a layer that sends events to AWS CloudWatch Logs.
 //!  - [`clippy-tracing`] provides a tool to add, remove and check for `tracing::instrument`.
+//!  - [`json-subscriber`] provides a subscriber for emitting JSON logs. The output can be customized much more than with [`tracing-subscriber`]'s JSON output.
 //!
 //! If you're the maintainer of a `tracing` ecosystem crate not listed above,
 //! please let us know! We'd love to add your project to the list!
@@ -860,6 +861,7 @@
 //! [`reqwest`]: https://crates.io/crates/reqwest
 //! [`tracing-cloudwatch`]: https://crates.io/crates/tracing-cloudwatch
 //! [`clippy-tracing`]: https://crates.io/crates/clippy-tracing
+//! [`json-subscriber`]: https://crates.io/crates/json-subscriber
 //!
 //! <div class="example-wrap" style="display:inline-block">
 //! <pre class="ignore" style="white-space:normal;font:inherit;">