docs: add diagnostics-channel plugin to core list (#2875)

docs/Ecosystem.md

@@ -20,6 +20,7 @@ Plugins maintained by the fastify team are listed under [Core](#core) while plug
 - [`fastify-cookie`](https://github.com/fastify/fastify-cookie) Parse and set cookie headers.
 - [`fastify-cors`](https://github.com/fastify/fastify-cors) Enables the use of CORS in a Fastify application.
 - [`fastify-csrf`](https://github.com/fastify/fastify-csrf) A plugin for adding [CSRF](https://en.wikipedia.org/wiki/Cross-site_request_forgery) protection to Fastify.
+- [`fastify-diagnostics-channel`](https://github.com/fastify/fastify-diagnostics-channel) Plugin to deal with `diagnostics_channel` on Fastify
 - [`fastify-elasticsearch`](https://github.com/fastify/fastify-elasticsearch) Plugin to share the same ES client.
 - [`fastify-env`](https://github.com/fastify/fastify-env) Load and check configuration.
 - [`fastify-etag`](https://github.com/fastify/fastify-etag) Automatically generate etags for HTTP responses.