Update misleading Javadocs for client side metrics.

Stopwatch metrics always convert nanoseconds to milliseconds when getLong() is called on them.

Author: avshenuk

client-v2/src/main/java/com/clickhouse/client/api/metrics/ClientMetrics.java

@@ -3,12 +3,12 @@
 public enum ClientMetrics {
 
     /**
-     * Operation duration in nanoseconds.
+     * Operation duration in milliseconds.
      */
     OP_DURATION("client.opDuration"),
 
     /**
-     * Duration of the operation serialization step in nanoseconds.
+     * Duration of the operation serialization step in milliseconds.
      */
     OP_SERIALIZATION("client.opSerialization");