[BUG] Behaviors of command `rare` and `top` are inconsistent with OpenSearch PPL

[LantaoJin]

What is the bug?
source = $testTable| rare address should return one field address

But got two fields in return.

+-------------+---------+
|count_address|  address|
+-------------+---------+
|            1|Vancouver|
|            2| Portland|
|            2|  Seattle|
+-------------+---------+

Similar issue in top command.

Related to #568

What is the expected behavior?
Only the field after rare/top command return.