[narwhal fix subscriber metrics (MystenLabs#8061)

Certificate specific metrics were placed into the loop that is
processing the certificate's batches. However, this doesn't allow us to
get signal when the certificate referencing no batches giving us wrong
impression in diagnostics.

narwhal/executor/src/subscriber.rs

@@ -235,14 +235,17 @@ impl<Network: SubscriberNetwork> Fetcher<Network> {
         for cert in &sub_dag.certificates {
             let mut batches = Vec::with_capacity(num_batches);
             let output_cert = cert.clone();
+
+            self.metrics
+                .subscriber_current_round
+                .set(cert.round() as i64);
+
+            self.metrics
+                .subscriber_certificate_latency
+                .observe(cert.metadata.created_at.elapsed().as_secs_f64());
+
             for (digest, (worker_id, _)) in cert.header.payload.iter() {
-                self.metrics
-                    .subscriber_current_round
-                    .set(cert.round() as i64);
                 self.metrics.subscriber_processed_batches.inc();
-                self.metrics
-                    .subscriber_certificate_latency
-                    .observe(cert.metadata.created_at.elapsed().as_secs_f64());
 
                 let mut workers = self.network.workers_for_certificate(cert, worker_id);