playground: fix tiproxy metrics addr

[xhebox]

What problem does this PR solve?

Playground promtheus pulls wrong metric addresses for tiproxy.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release notes:

Fix tiproxy metrics on playground

[codecov-commenter]

Codecov Report

Attention: 8 lines in your changes are missing coverage. Please review.

Comparison is base (721b065) 55.47% compared to head (7448ca0) 55.43%.

❗ Current head 7448ca0 differs from pull request most recent head 76a461b. Consider uploading reports for the commit 76a461b to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2299      +/-   ##
==========================================
- Coverage   55.47%   55.43%   -0.04%     
==========================================
  Files         326      326              
  Lines       34479    34478       -1     
==========================================
- Hits        19125    19111      -14     
- Misses      13063    13075      +12     
- Partials     2291     2292       +1     

Flag	Coverage Δ
playground	15.36% <61.90%> (-0.07%)	⬇️
tiup	15.67% <ø> (ø)
unittest	22.43% <0.00%> (-<0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
components/playground/instance/instance.go	44.29% <100.00%> (ø)
components/playground/playground.go	44.00% <100.00%> (-0.10%)	⬇️
components/playground/monitor.go	76.67% <81.82%> (-4.39%)	⬇️
components/playground/instance/tiproxy.go	0.00% <0.00%> (ø)

... and 1 file with indirect coverage changes

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-11-03 07:01:27.271763579 +0000 UTC m=+3195684.858873724: ☑️ agreed by kaaaaaaang.

[xhebox]

/approve

[kaaaaaaang]

/lgtm

[kaaaaaaang]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: kaaaaaaang, xhebox

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