refactor(layers/logging): parsing level str

[PsiACE]

Attempt to parse str as level and parse level must succeed.

fixes: #2090

[xyjixyjixyji]

I think we can directly return Self instead of Result<Self> since any error happens here will lead to panic.

[Xuanwo]

I think we can directly return Self instead of Result<Self> since any error happens here will lead to panic.

We should not panic here. It's possible that users can input invalid values like Haha. We should return an error instead.

[Xuanwo]

Thanks!