Router support set fallback route as downgrading routing strategy

[chengyouling]

What would you like to be enhanced?

Router add fallback rule to support downgrading routing strategy.

Why is this needed?

Support multi permanent path routing degradation strategy.

[provenceee]

标签路由目前存在以下问题，当存在多标签的实例时，有可能存在以下的配置：

- precedence: 0
  route:
  - weight: 100
    tags:
      cas_lane_tag: base
    weight: 100
- precedence: 1
  match:
    headers:
      x-cse-canary:
        exact: red
  route:
  - weight: 100
    tags:
      cas_lane_tag: red
- precedence: 2
  match:
    headers:
      x-cse-canary:
        exact: gray
  route:
  - weight: 100
    tags:
      cas_lane_tag: gray

那么会存在一种场景，在全链路灰度的情况下，如果一个请求匹配上了cas_lane_tag: gray的路由，但是却没有部署实例，那么将会根据负载均衡策略选取实例，也就说有可能会路由到red组的实例或者base组的实例，但大多数场景下，更多的是希望能路由到base组，所以需要增加fallback机制，配置可拓展为：

- precedence: 0
  route:
  - weight: 100
    tags:
      cas_lane_tag: base
    weight: 100
- precedence: 1
  match:
    headers:
      x-cse-canary:
        exact: red
  route:
  - weight: 100
    tags:
      cas_lane_tag: red
  fallback:
  - tags:
      cas_lane_tag: base
    weight: 100
- precedence: 2
  match:
    headers:
      x-cse-canary:
        exact: gray
  route:
  - weight: 100
    tags:
      cas_lane_tag: gray
  fallback:
  - tags:
      cas_lane_tag: base
    weight: 100

那么，没有red组或者gray组的情况下，将fallback到base组，解决了之前的不足