placement: add peer healthy factor when comparing region fit

[nolouch]

What problem does this PR solve?

Issue Number: Close #6559

When Region looks like:

{
  "id": 55929554,
  "start_key": "748000000000000BFF065F728000000042FFEF8B7A0000000000FA",
  "end_key": "748000000000000BFF065F728000000042FFF07AFD0000000000FA",
  "epoch": {
    "conf_ver": 6,
    "version": 109399
  },
  "peers": [
    {
      "id": 55929555,
      "store_id": 1,
      "role_name": "Voter"
    },
    {
      "id": 55929556,
      "store_id": 4,
      "role_name": "Voter"
    },
    {
      "id": 55929557,
      "store_id": 5,
      "role_name": "Voter"
    },
    {
      "id": 55929558,
      "store_id": 2751139,
      "role": 1,
      "role_name": "Learner",
      "is_learner": true
    }
  ],
  "leader": {
    "id": 55929555,
    "store_id": 1,
    "role_name": "Voter"
  },
  "down_peers": [
    {
      "down_seconds": 40307,
      "peer": {
        "id": 55929556,
        "store_id": 4,
        "role_name": "Voter"
      }
    }
  ],
  "pending_peers": [
    {
      "id": 55929556,
      "store_id": 4,
      "role_name": "Voter"
    }
  ],
  "cpu_usage": 0,
  "written_bytes": 0,
  "read_bytes": 0,
  "written_keys": 0,
  "read_keys": 0,
  "approximate_size": 1,
  "approximate_keys": 40960
}

and the region fit likes:

{
  "rule-fits": [
    {
      "rule": {
        "group_id": "pd",
        "id": "default",
        "start_key": "",
        "end_key": "",
        "role": "voter",
        "is_witness": false,
        "count": 3,
        "location_labels": [
          "region",
          "zone",
          "host"
        ]
      },
      "peers": [
        {
          "id": 55929555,
          "store_id": 1
        },
        {
          "id": 55929557,
          "store_id": 5
        },
        {
          "id": 55929556,
          "store_id": 4
        }
      ],
      "peers-different-role": null,
      "isolation-score": 300
    }
  ],
  "orphan-peers": [
    {
      "id": 55929558,
      "store_id": 2751139,
      "role": 1
    }
  ]
}

cannot fix the region, always report as an unhealthy region. and store 4 is gone.

What is changed and how does it work?

 add peer healthy factor when comparing region fit

Check List

Tests

• Unit test

Release note

None.

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[codecov]

Codecov Report

Merging #6822 (8e4aeb6) into master (40eaa35) will increase coverage by 0.02%.
The diff coverage is 89.18%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6822      +/-   ##
==========================================
+ Coverage   74.15%   74.18%   +0.02%     
==========================================
  Files         413      413              
  Lines       43303    43309       +6     
==========================================
+ Hits        32112    32128      +16     
+ Misses       8324     8318       -6     
+ Partials     2867     2863       -4     

Flag	Coverage Δ
unittests	74.18% <89.18%> (+0.02%)	⬆️

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[nolouch]

#6831 is another way that introduces fewer changes, so close this one.