Use a fixed metric name for sentinel prometheus exporter

### Issue Description

**Type:** *Enhancement*

I have followed this [README](https://github.com/alibaba/Sentinel/blob/a524ab3bb3364818e292e1255480d20845e77c89/sentinel-extension/sentinel-prometheus-metric-exporter/README.md) to set up sentinel prometheus exporter. However, I've observed that the metric name is currently tied to the `appName`, which is typically the JAR's name or specified by the VM option `csp.sentinel.prometheus.app`. The logic can be found at [SentinelCollector.java#L83](https://github.com/alibaba/Sentinel/blob/master/sentinel-extension/sentinel-prometheus-metric-exporter/src/main/java/com/alibaba/csp/sentinel/metric/prom/collector/SentinelCollector.java#L83).

Querying metrics using `appName` in PromQL seems unconventional and inconvenient. In scenarios where multiple services enable the Prometheus exporter feature, prometheus will receive numerous distinct metrics that essentially convey the same information.

I propose using a fixed metric name and introducing an additional label (`app_name`) representing the `appName`.

### Desired Feature

I would like the Prometheus exporter to provide a fixed metric name and an extra label (`app_name`). This would enable querying with a consistent PromQL expression like `sentinel_metric_log{app_name="xx", resource="xxx", type="xxx"}`. With this enhancement, every service can be queried using the same PromQL, requiring only the label's value to be adjusted.