`--path-prefix` not working as expected

[mornyx]

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Bug Report

What did you do?

1. Build tidb-dashboard binary.

SKIP_GEN_API=1 make package

2. Run TiDB cluster.

tiup playground

3. Run standalone tidb-dashboard with --path-prefix.

./bin/tidb-dashboard --debug --experimental --feature-version 999.999.999 --host 0.0.0.0 --path-prefix=/test

4. Access http://localhost:12333/test

What did you expect to see?

TiDB dashboard login page serve on http://localhost:12333/test.

What did you see instead?

Jump back to http://localhost:12333/dashboard and load the page forever.

What version of TiDB Dashboard are you using (./tidb-dashboard --version)?

master branch