Use readLock in `getActiveRatio`

server/src/main/java/org/apache/iotdb/db/conf/adapter/ActiveTimeSeriesCounter.java

@@ -120,7 +120,7 @@ public class ActiveTimeSeriesCounter implements IActiveTimeSeriesCounter {
 
   @Override
   public double getActiveRatio(String storageGroup) {
-    lock.writeLock().lock();
+    lock.readLock().lock();
     double ratio;
     try {
       ratio = activeRatioMap.get(storageGroup);
@@ -128,7 +128,7 @@ public class ActiveTimeSeriesCounter implements IActiveTimeSeriesCounter {
       ratio = 0;
       LOGGER.error("Get active ratio failed", e);
     } finally {
-      lock.writeLock().unlock();
+      lock.readLock().unlock();
     }
     return ratio;
   }
@@ -156,4 +156,4 @@ public class ActiveTimeSeriesCounter implements IActiveTimeSeriesCounter {
     activeRatioMap = new ConcurrentHashMap<>();
     activeTimeSeriesNumMap = new ConcurrentHashMap<>();
   }
-}
\ No newline at end of file
+}