Add tokio thread number in the default config file.

DEFAULT_CONFIG.json5

@@ -79,6 +79,24 @@
       //   // root_ca_certificate_base64: "base64-root-ca-certificate",
       // },
 
+      ////
+      //// This plugin uses Tokio (https://tokio.rs/) for asynchronous programming. 
+      //// When running as a plugin within a Zenoh router, the plugin creates its own Runtime managing 2 pools of threads:
+      ////    - worker threads for non-blocking tasks. Those threads are spawn at Runtime creation.
+      ////    - blocking threads for blocking tasks (e.g. I/O). Those threads are spawn when needed.
+      //// For more details see https://github.com/tokio-rs/tokio/discussions/3858#discussioncomment-869878
+      //// When running as a standalone bridge the Zenoh Session's Runtime is used and can be configured via the
+      //// `ZENOH_RUNTIME` environment variable. See https://docs.rs/zenoh-runtime/latest/zenoh_runtime/enum.ZRuntime.html
+      ////
+
+      //// work_thread_num: The number of worker thread in the asynchronous runtime will use. (default: 2)
+      ////                  Only for a plugin, no effect on a bridge.
+      // work_thread_num: 2,
+
+      //// max_block_thread_num: The number of blocking thread in the asynchronous runtime will use. (default: 50)
+      ////                       Only for a plugin, no effect on a bridge.
+      // max_block_thread_num: 50,
+
       ////
       //// MQTT client authentication related configuration.
       ////

zenoh-plugin-mqtt/src/lib.rs

@@ -36,7 +36,7 @@ use rustls::{
 };
 use secrecy::ExposeSecret;
 use serde_json::Value;
-use tokio::{runtime::Handle, task::JoinHandle};
+use tokio::task::JoinHandle;
 use zenoh::{
     bytes::{Encoding, ZBytes},
     internal::{