`closest-adaptive` performs replica read in leader

[you06]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

mysql -u root -h test-tongmu-tidb-0 -P 4000 -e "set global tidb_replica_read = 'closest-adaptive';"
sleep 5
sysbench --config-file=config oltp_point_select --tables=1 --table-size=100000000 --auto_inc=true run

2. What did you expect to see? (Required)

The performance of closest-adaptive is close to leader read.

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

nightly