feat(observability): export rate and load metrics from rate controller

Send rates, load statistics, and delay deltas via the metrics channel to enable real-time monitoring of network performance.

src/ratecontroller.rs

@@ -409,6 +409,18 @@ impl Ratecontroller {
                     self.state_ul.current_rate as u64
                 );
 
+                if let Some(ref tx) = self.metrics_tx {
+                    let _ = tx.try_send(Metric::Rate {
+                        dl_rate: self.state_dl.current_rate,
+                        ul_rate: self.state_ul.current_rate,
+                        rx_load: self.state_dl.load,
+                        tx_load: self.state_ul.load,
+                        delta_delay_down: self.state_dl.delta_stat,
+                        delta_delay_up: self.state_ul.delta_stat,
+                        timestamp_ns: stats_time.as_nanos(),
+                    });
+                }
+
                 if let Some(ref mut fd) = stats_fd {
                     if let Err(e) = fd.write_all(
                         format!(