Incorrect clientWriteRequest timer usage

clientWriteRequest timer measures the time from receiving the client request until the leader responds back.

But this timer is only initialized when the leader responds, so the timer is always close to zero.
https://github.com/apache/ratis/blob/branch-2/ratis-server/src/main/java/org/apache/ratis/server/impl/RaftServerImpl.java#L840

The bug is accidentally fixed in 3.0.0 by RATIS-1693. But for users of 2.x this may still be worth fixing.