add engine config

Former-commit-id: 326501e3adc70b88fe3b8304ba39920fb11cf2d0

cpp/src/server/ServerConfig.h

@@ -43,6 +43,9 @@ static const std::string CONFIG_METRIC_COLLECTOR = "collector";
 static const std::string CONFIG_PROMETHEUS = "prometheus_config";
 static const std::string CONFIG_METRIC_PROMETHEUS_PORT = "port";
 
+static const std::string CONFIG_ENGINE = "engine_config";
+static const std::string CONFIG_NPROBE = "nprobe";
+
 class ServerConfig {
  public:
     static ServerConfig &GetInstance();