docs(ecosystem): add @fastify/otel to core list (fastify#5967)

Author: Fdawgs

docs/Guides/Ecosystem.md

@@ -86,6 +86,8 @@ section.
   [`simple-oauth2`](https://github.com/lelylan/simple-oauth2).
 - [`@fastify/one-line-logger`](https://github.com/fastify/one-line-logger) Formats
   Fastify's logs into a nice one-line message.
+- [`@fastify/otel`](https://github.com/fastify/otel) OpenTelemetry
+  instrumentation library.
 - [`@fastify/passport`](https://github.com/fastify/fastify-passport) Use Passport
   strategies to authenticate requests and protect route.
 - [`@fastify/postgres`](https://github.com/fastify/fastify-postgres) Fastify