More plentiful "snapshot" value in circuit breaker observer functions

[NineSunRD]

Issue Description

Type: feature request

Describe what feature you want

Snapshot is interface，I think it is for more plentiful data，but we only pass a simple param when we use it，I think we can pass more information，such as cur state，and we can get more information in the listener.

Additional context

Add any other context or screenshots about the feature request here.

[sczyh30]

Hi, we've already carried previous state, triggered rule and "snapshot" value (if needed) in the observer functions. You may refer to the example: https://github.com/alibaba/sentinel-golang/blob/master/example/circuitbreaker/circuit_breaker_example.go

[NineSunRD]

Thanks very much

the cur state that I mean is not the open state or half open state, I mean the cur metrics, such as cur request number or cur rt. because I want to do some alert with cur info when system is breaking

on the other hand，I have reviewed the code in core/log/metric , because we have our own monitor system，I suggest the metric need easier to access to other systems

[sczyh30]

the cur state that I mean is not the open state or half open state, I mean the cur metrics, such as cur request number or cur rt. because I want to do some alert with cur info when system is breaking

Currently only the triggered ratio was provided. We may improve the "snapshot" value later so that it could provide more useful metrics.

on the other hand，I have reviewed the code in core/log/metric , because we have our own monitor system，I suggest the metric need easier to access to other systems

We're working on it. We may discuss it in #70

[NineSunRD]

Thanks