rule: let default rule fit the region at last

[Yisaer]

What problem does this PR solve?

Given region and store with following status:

// Stores:     1    2    3    4
// Leaders:    1    0    0    0
// Region1:    L    F    F    F

Then post a rule like following:

  {
    "group_id": "test",
    "id": "test-leader",
    "index": 1,
    "start_key": "",
    "end_key": "",
    "role": "leader",
    "count": 1,
    "label_constraints": [
      {
        "key": "role",
        "op": "in",
        "values": [
          "leader"
        ]
      }
    ],
    "location_labels": [
      "host"
    ]
  }

Then label store 1 as "role": "leader"

The expected RegionFit is:

For rule "test", the peers are peer-1 in store 1
For rule "pd", the peers are [peer-2,peer-3,peer-4] in store2/3/4

What we got is:

For rule "test", the peers are empty.
For rule "pd", the peers are [peer-1,peer-2,peer-3] in store 1/2/3

What is changed and how it works?

One possible solution is to let default rule fit the region at last.

Check List

Tests

• Unit test

Release note

[lhy1024]

LGTM

[disksing]

I've composed another fix so closing it.