[fix](memtracker) PushBrokerReader mem_pool has no tracker to control memory

[xinyiZzz]

Proposed changes

Issue Number: close #xxx

Problem summary

#0  doris::MemTracker::consumption (this=0x0) at /root/doris/be/src/runtime/memory/mem_tracker.h:65
#1  doris::PushHandler::_convert_v2 (this=this@entry=0x7ffec1e17f90, cur_tablet=..., cur_rowset=cur_rowset@entry=0x7ffec1e177a0, tablet_schema=...)
    at /root/doris/be/src/olap/push_handler.cpp:247
#2  0x000055555a271bc6 in doris::PushHandler::_do_streaming_ingestion (this=this@entry=0x7ffec1e17f90, tablet=..., request=...,
    push_type=push_type@entry=doris::PUSH_NORMAL_V2, tablet_info_vec=tablet_info_vec@entry=0x7ffec1e185d0)
    at /var/local/ldb-toolchain/include/c++/11/ext/atomicity.h:109
#3  0x000055555a27222c in doris::PushHandler::process_streaming_ingestion

Only for 1.2, master does not have this issue

Checklist(Required)

• Does it affect the original behavior
• Has unit tests been added
• Has document been added or modified
• Does it need to update dependencies
• Is this PR support rollback (If NO, please explain WHY)

Further comments

If this is a relatively large or complex change, kick off the discussion at [email protected] by explaining why you chose the solution you did and what alternatives you considered, etc...

[yiguolei]

LGTM

[morningman]

LGTM

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.