mcs: fix duplicated metrics

[rleungx]

What problem does this PR solve?

Issue Number: Close #7290.

What is changed and how does it work?

This PR moves redirector after root group init so that the metrics request won't be redirected to the primary.

Check List

Tests

• Manual test

tiup playground nightly --pd.mode ms --pd.api 1 --pd.scheduling 2 --kv 3 --tiflash 0 --host 10.2.8.101

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• JmPotato
• lhy1024

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[rleungx]

/check-issue-triage-complete

[codecov]

Codecov Report

Merging #7319 (f754ae1) into master (689fcbe) will decrease coverage by 0.17%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #7319      +/-   ##
==========================================
- Coverage   74.52%   74.35%   -0.17%     
==========================================
  Files         446      446              
  Lines       48346    48346              
==========================================
- Hits        36030    35950      -80     
- Misses       9146     9206      +60     
- Partials     3170     3190      +20     

Flag	Coverage Δ
unittests	74.35% <100.00%> (-0.17%)	⬇️

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[rleungx]

/merge

[ti-chi-bot]

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[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: f754ae1