statistics: get region info via core cluster inside RegionStatistics (#6804)

[ti-chi-bot]

This is an automated cherry-pick of #6804

What problem does this PR solve?

Issue Number: Close #6560.

What is changed and how does it work?

Instead of maintaining its own region info cache, use the core cluster to
fetch the region info inside `RegionStatistics` to make sure consistency.

Check List

Tests

• Unit test
• Integration test
• Manual test

Run pd-ctl region check miss-peer:

[
  {
      "id": 22,
      "start_key": "7480000000000000FF5C00000000000000F8",
      "end_key": "748000FFFFFFFFFFFFF900000000000000F8",
      "epoch": {
        "conf_ver": 1,
        "version": 56
      },
      "peers": [
        {
          "id": 23,
          "store_id": 1,
          "role_name": "Voter"
        }
      ],
      "leader": {
        "id": 23,
        "store_id": 1,
        "role_name": "Voter"
      },
      "cpu_usage": 0,
      "written_bytes": 204,
      "read_bytes": 0,
      "written_keys": 4,
      "read_keys": 0,
      "approximate_size": 1,
      "approximate_keys": 0
  }
]

Run pd-ctl region 22:

{
  "id": 22,
  "start_key": "7480000000000000FF5C00000000000000F8",
  "end_key": "748000FFFFFFFFFFFFF900000000000000F8",
  "epoch": {
    "conf_ver": 1,
    "version": 56
  },
  "peers": [
    {
      "id": 23,
      "store_id": 1,
      "role_name": "Voter"
    }
  ],
  "leader": {
    "id": 23,
    "store_id": 1,
    "role_name": "Voter"
  },
  "cpu_usage": 0,
  "written_bytes": 204,
  "read_bytes": 0,
  "written_keys": 4,
  "read_keys": 0,
  "approximate_size": 1,
  "approximate_keys": 0
}

Release note

None.

[ti-chi-bot]

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Adding the do-not-merge/cherry-pick-not-approved label.

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[nolouch]

no need for 5.x