feat(layer/prometheus): Support output path as a metric label

[Zheaoli]

For now, we already have basic metric output in the Prometheus layer.

1. requests_total
2. requests_duration_seconds
3. bytes_total

All the metrics are grouped by scheme and operation for now.

For now, in some circumstances, we need to group by the metric by an extra path. Here's an example: if we choose to write to different devices in Linux using OpenDAL, and one of the devices has some problem, the write and read action becomes slow and slow. We can't monitor and alert for the specific device if we don't have a path label.

But on the other hand, if we output all the paths as a label in metric, the final metric we get will be extremely massive. So I chose to introduce a new config option for the people to choose how many path levels they want to export

[Xuanwo]

Thanks!