lalb try left server nodes

[FancyJan]

What problem does this PR solve?

Issue Number:
#2427
Problem Summary:
lalb在有可用节点的情况下仍可能触发“[E112]Fail to select server...”

What is changed and the side effects?

Changed:

Side effects:

• Performance effects(性能影响):
  可能会增加lalb的长尾
• Breaking backward compatibility(向后兼容性):

---

Check List:

• Please make sure your changes are compilable(请确保你的更改可以通过编译).
• When providing us with a new feature, it is best to add related tests(如果你向我们增加一个新的功能, 请添加相关测试).
• Please follow Contributor Covenant Code of Conduct.(请遵循贡献者准则).

[wwbmmm]

LGTM

[chenBright]

LGTM