feat: add metrics for LND tracker

Add gauges to monitor the LND tracker's internal state for better observability.

- Track invoice queue length to monitor backlog size
- Monitor count of active invoice trackers for capacity awareness
- Conditionally compile metrics behind "metrics" feature flag

Author: doitian

crates/fiber-lib/src/cch/trackers/lnd_trackers.rs

@@ -240,7 +240,7 @@ impl Actor for LndTrackerActor {
         message: Self::Msg,
         state: &mut Self::State,
     ) -> Result<(), ActorProcessingErr> {
-        match message {
+        let res = match message {
             LndTrackerMessage::TrackInvoice(payment_hash) => {
                 state.invoice_queue.push_back(payment_hash);
                 state.process_invoice_queue(myself).await?;
@@ -278,7 +278,18 @@ impl Actor for LndTrackerActor {
                 let _ = reply_port.send(snapshot);
                 Ok(())
             }
+        };
+
+        // update metrics
+        #[cfg(feature = "metrics")]
+        {
+            metrics::gauge!(crate::metrics::CCH_LND_TRACKER_INVOICE_QUEUE_LEN)
+                .set(state.invoice_queue.len() as f64);
+            metrics::gauge!(crate::metrics::CCH_LND_TRACKER_ACTIVE_INVOICE_TRACKERS)
+                .set(state.active_invoice_trackers as f64);
         }
+
+        res
     }
 }
 

crates/fiber-lib/src/metrics.rs

@@ -10,6 +10,10 @@ pub const INBOUND_PEER_COUNT: &str = "fiber.inbound_peer_count";
 pub const OUTBOUND_PEER_COUNT: &str = "fiber.outbound_peer_count";
 pub const DOWN_WITH_CHANNEL_PEER_COUNT: &str = "fiber.down_with_channel_peer_count";
 
+pub const CCH_LND_TRACKER_INVOICE_QUEUE_LEN: &str = "fiber.cch.lnd_tracker.invoice_queue_len";
+pub const CCH_LND_TRACKER_ACTIVE_INVOICE_TRACKERS: &str =
+    "fiber.cch.lnd_tracker.active_invoice_trackers";
+
 pub fn start_metrics(metrics_addr: &str) -> Result<(), Box<dyn std::error::Error>> {
     let socket_addr = metrics_addr
         .to_socket_addrs()