export an open-todos gauge and chart it on the agent dashboard

hive-agent/src/todos_gauge.rs

@@ -0,0 +1,25 @@
+//! Feeds the count of open (un-acked) todos into an OTEL gauge on a timer.
+//! Todos arrive and leave between turns, so the per-turn record would
+//! leave the series stale while the agent sleeps.
+
+use std::sync::Arc;
+use std::time::Duration;
+
+use crate::todos::Todos;
+
+/// Half the exporter's one-minute interval, so every export carries a
+/// value at most one tick old.
+const TICK: Duration = Duration::from_secs(30);
+
+/// Background loop: record the open-todo count every [`TICK`]. Detached
+/// task — runs for the harness's lifetime; read errors are logged, never
+/// fatal.
+pub async fn run(todos: Arc<Todos>) {
+    loop {
+        match todos.open_count() {
+            Ok(n) => crate::otel_turn_metrics::record_open_todos(n),
+            Err(e) => tracing::warn!(error = %e, "open todo count failed"),
+        }
+        tokio::time::sleep(TICK).await;
+    }
+}