Could metric exclude resources those are not in rules?

## Issue Description

Type: *feature request*

### Describe what happened (or what feature you want)
We are using sentinel, while we collected the metric, we found that all request will be recorded in the **metric.log. In our scenes, we hope the resource those are not in our rules may not be recorded, in case to reduce metrics.
我们公司当前正在使用sentinel，我们发现在收集metric的时候，所有的请求都会被统计在 metric.log中，不论资源是否是限流或者熔断或者是未配置的资源。
我们希望有配置能够让非配置的资源不进入metric统计，来实现减少metric的目标。当前统计所有接口的话metric实在太多了。
我们当前使用的版本是1.8.0，查看了release note，没有发现满足上述需求的内容。

### Describe what you expected to happen
The resource those are not in our rules may not be recorded, in case to reduce metrics.
我们希望有配置能够让非配置的资源不进入metric统计，来实现减少metric的目标。

### How to reproduce it (as minimally and precisely as possible)

1. start the app with sentinel
2. visit a resource that not in rules
3. the visit records will be in metric.log

1. 启动接入sentinel的应用
2. 访问一个未配置任何限流或者降级规则的资源（接口）
3. 该资源的访问被记录在metric.log中

### Tell us your environment
sdk version 1.8.0

### Anything else we need to know?
No

Start by tracing how Sentinel SDK 1.8.0 records resources in metric.log and where rule presence is checked. Confirm the behavior with the three-step reproduction using a resource without flow-control or degradation rules. Done means unconfigured resources can be excluded from metric recording while configured resources remain recorded.

Written by the indexing model from the issue text.