Internal metrics support.

## Feature Request

### Crates

volo-otel? volo-var?

### Motivation

like bvar in brpc. support export to otel or openmetrics format to prometheus easy.

export runtime metrics, like eventloop delay.

### Proposal

like bvar. write metrics var to thread local storage. and merge all tls on read.

### Alternatives
if not impl it in framework. user may need do it on application layer and hard to monitor runtime metrics. may conflict with volo's better perf. such like some prom client use one instance and need mutex. it may conflict with volo's slogan lock free.