[DISCUSSION] Rate limiting and fault-tolerance spec | 流控降级与容错标准讨论

## Issue Description

Type: *feature request*

### Describe what happened (or what feature you want)

Hi community, we've extracted a draft of specification from Sentinel about rate limiting and fault-tolerance spec, and we've put the draft into [the OpenSergo spec](https://github.com/opensergo/opensergo-specification).

Refer https://github.com/opensergo/opensergo-specification/issues/8

Discussions are welcomed.

Start by reading the OpenSergo specification and the referenced OpenSergo issue 8, then review how the draft relates to Sentinel's rate-limiting and fault-tolerance concerns. This issue is a discussion rather than a defined implementation task; done would require an agreed specification or concrete scope.

Written by the indexing model from the issue text.