Add RecentItems metrics (#20484) (#20490)

rpc/src/rpc_subscriptions.rs

@@ -213,6 +213,12 @@ impl RecentItems {
                 .checked_sub(item.len())
                 .expect("total bytes underflow");
         }
+
+        datapoint_info!(
+            "rpc_subscriptions_recent_items",
+            ("num", self.queue.len(), i64),
+            ("total_bytes", self.total_bytes, i64),
+        );
     }
 }