Bring back queue length stats.

Author: jsdt

crates/core/src/host/module_host.rs

@@ -653,6 +653,17 @@ impl ModuleHost {
             .reducer_wait_time
             .with_label_values(&self.info.database_identity, reducer)
             .start_timer();
+        let queue_length_gauge = WORKER_METRICS
+            .instance_queue_length
+            .with_label_values(&self.info.database_identity);
+        queue_length_gauge.inc();
+        {
+            let queue_length = queue_length_gauge.get();
+            WORKER_METRICS
+                .instance_queue_length_histogram
+                .with_label_values(&self.info.database_identity)
+                .observe(queue_length as f64);
+        }
 
         // Operations on module instances (e.g. calling reducers) is blocking,
         // partially because the computation can potentialyl take a long time
@@ -671,6 +682,7 @@ impl ModuleHost {
             .job_tx
             .run(move |inst| {
                 queue_timer.stop_and_record();
+                queue_length_gauge.dec();
                 f(inst)
             })
             .await;