Fix update logic of minRt in bucket

- Fix incorrect usage. It's not thread-safe when comparing and updating `minRt`, but it's okay since the actual minimum RT often matches free load.

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/statistic/base/Window.java

@@ -15,6 +15,8 @@
  */
 package com.alibaba.csp.sentinel.slots.statistic.base;
 
+import com.alibaba.csp.sentinel.Constants;
+
 /**
  * Represents metrics data in a period of time window.
  *
@@ -28,14 +30,15 @@ public class Window {
     private final LongAdder exception = new LongAdder();
     private final LongAdder rt = new LongAdder();
     private final LongAdder success = new LongAdder();
-    private final LongAdder minRt = new LongAdder();
+
+    private volatile long minRt;
 
     public Window() {
         initMinRt();
     }
 
     private void initMinRt() {
-        minRt.add(4900);
+        this.minRt = Constants.TIME_DROP_VALVE;
     }
 
     /**
@@ -49,7 +52,6 @@ public class Window {
         exception.reset();
         rt.reset();
         success.reset();
-        minRt.reset();
         initMinRt();
         return this;
     }
@@ -71,7 +73,7 @@ public class Window {
     }
 
     public long minRt() {
-        return minRt.longValue();
+        return minRt;
     }
 
     public long success() {
@@ -97,8 +99,9 @@ public class Window {
     public void addRT(long rt) {
         this.rt.add(rt);
 
-        if (minRt.longValue() > rt) {
-            minRt.internalReset(rt);
+        // Not thread-safe, but it's okay.
+        if (rt < minRt) {
+            minRt = rt;
         }
     }
 }

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/statistic/metric/ArrayMetric.java

@@ -18,6 +18,7 @@ package com.alibaba.csp.sentinel.slots.statistic.metric;
 import java.util.ArrayList;
 import java.util.List;
 
+import com.alibaba.csp.sentinel.Constants;
 import com.alibaba.csp.sentinel.node.metric.MetricNode;
 import com.alibaba.csp.sentinel.slots.statistic.base.Window;
 import com.alibaba.csp.sentinel.slots.statistic.base.WindowWrap;
@@ -117,7 +118,7 @@ public class ArrayMetric implements Metric {
     @Override
     public long minRt() {
         data.currentWindow();
-        long rt = 4900;
+        long rt = Constants.TIME_DROP_VALVE;
         List<Window> list = data.values();
         for (Window window : list) {
             if (window.minRt() < rt) {