fix: gracefully close the instantaneous_metric thread

Gracefully close the instantaneous_metric thread.

Fixes: #1756

Signed-off-by: yaoyinnan <yaoyinnan@foxmail.com>

Author: yaoyinnan

src/pika_server.cc

@@ -206,6 +206,12 @@ void PikaServer::Start() {
       exit_mutex_.unlock();
     }
   }
+
+  LOG(INFO) << "Instantaneous metric thread exit";
+  if (instantaneous_metric.joinable()) {
+    instantaneous_metric.join();
+  }
+
   LOG(INFO) << "Goodbye...";
 }