MDEV-19822 MariaDB 10.4 install fails on Ubuntu 18.04 if Prometheus mysqld_exporter is running

debian/mariadb-server-10.4.preinst

@@ -22,7 +22,7 @@ mysql_upgradedir=/var/lib/mysql-upgrade
 stop_server() {
     # Return immediately if there are no mysql processes running
     # as there is no point in trying to shutdown in that case.
-    if ! pgrep mysqld > /dev/null; then return; fi
+    if ! pgrep -x mysqld > /dev/null; then return; fi
 
     set +e
     systemctl stop mysql