Add getCpuUsageThreshold() method in SystemRuleManager (#770)

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/system/SystemRuleManager.java

@@ -241,6 +241,10 @@ public class SystemRuleManager {
     public static void setHighestSystemLoad(double highestSystemLoad) {
         SystemRuleManager.highestSystemLoad = highestSystemLoad;
     }
+    
+    public static double getCpuUsageThreshold() {
+        return highestCpuUsage;
+    }
 
     public static void loadSystemConf(SystemRule rule) {
         boolean checkStatus = false;
@@ -343,4 +347,4 @@ public class SystemRuleManager {
     public static double getCurrentCpuUsage() {
         return statusListener.getCpuUsage();
     }
-}
+}