PD should consider the available storage size to detect low space store

## Feature Request

### Describe your feature request related problem

Currently, PD detected low space store by low-space-ratio (default 0.8), if a store has a large storage capacity, for example 2T or more, PD will detect such store as low space store even it has 400G available storage space which is unlikely will be filled up in a short time

### Describe the feature you'd like

PD should consider the available storage size to detect low space store

### Describe alternatives you've considered

PD dynamically adjust low-space-ratio, for example, when the storage capacity is 1T or less set the low-space-ratio to 0.8, when the storage capacity is 2T set the low-space-ratio to 0.9.

### Teachability, Documentation, Adoption, Migration Strategy

Stores with large storage capacity will not be detected as low space store when they still have quite a lot available storage space