Include rate limiting documentation

[20k-ultra]

What is the problem your feature solves, or the need it fulfills?

I am interested to use Pingora but without digging through source code I am unsure if it can rate limit various metrics as I hope. I want to enforce limits such as:

• RPS per downstream connection
• RPS per upstream host (basically a sum of downstream RPS)
• Total downstream connections for a given upstream host
• Total downstream connections for an IP

Describe the solution you'd like

I would like to know when and what I can rate limit. I've read the life of a request so understand there are several points I can hook into. I was just hoping the pingora-limits crate would help me out and have some examples or documentation without having to read the source.

Describe alternatives you've considered

Reading source code.

[20k-ultra]

if anyone knowledge-able with the source that knows the answer, I'd appreciate some input. I can figure out examples if it is possible. Will try to give it a look later.

[xialeistudio]

@20k-ultra I have submmited a PR to demonstrate how to use RateLimiter, which uses the request header-based dimension, you can implement your dimension by yourself.