measure how many changes get dropped

crates/corro-agent/src/agent/handlers.rs

@@ -888,6 +888,7 @@ pub async fn handle_changes(
 
         // drop old items when the queue is full.
         if queue.len() >= max_queue_len {
+            let mut dropped_count = 0;
             if let Some((dropped_change, _, _)) = queue.pop_front() {
                 for v in dropped_change.versions() {
                     if let Entry::Occupied(mut entry) = seen.entry((change.actor_id, v)) {
@@ -900,7 +901,9 @@ pub async fn handle_changes(
                 }
 
                 buf_cost -= dropped_change.processing_cost();
+                dropped_count += 1;
             }
+            counter!("corro.agent.changes.dropped").increment(dropped_count);
 
             log_at_pow_10("dropped old change from queue", &mut drop_log_count);
         }