feat: Fix typo in FlowRuleManager and improve code with Map.computeIfAbsent in MetricTimerListener (#2110)

sentinel-core/src/main/java/com/alibaba/csp/sentinel/node/metric/MetricTimerListener.java

@@ -62,9 +62,7 @@ public class MetricTimerListener implements Runnable {
             MetricNode metricNode = entry.getValue();
             metricNode.setResource(node.getName());
             metricNode.setClassification(node.getResourceType());
-            if (maps.get(time) == null) {
-                maps.put(time, new ArrayList<MetricNode>());
-            }
+            maps.computeIfAbsent(time, k -> new ArrayList<MetricNode>());
             List<MetricNode> nodes = maps.get(time);
             nodes.add(entry.getValue());
         }

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/block/flow/FlowRuleManager.java

@@ -77,7 +77,7 @@ public class FlowRuleManager {
     private static void startMetricTimerListener() {
         long flushInterval = SentinelConfig.metricLogFlushIntervalSec();
         if (flushInterval <= 0) {
-            RecordLog.info("[FlowRuleManager] The MetricTimerListener is'n started. If you want to start it, "
+            RecordLog.info("[FlowRuleManager] The MetricTimerListener isn't started. If you want to start it, "
                     + "please change the value(current: {}) of config({}) more than 0 to start it.", flushInterval,
                     SentinelConfig.METRIC_FLUSH_INTERVAL);
             return;