Collect metrics on scrape instead of timer

The prometheus docs on exporters say:

Metrics should only be pulled from the application when Prometheus scrapes them, exporters should not perform scrapes based on their own timers. That is, all scrapes should be synchronous.

The very first example provided by this repository goes against those docs by just spamming the same function on a timer.

Can anyone provide a basic example that shows how to start a metrics collection only when prometheus connects to the exporter, as it's actually supposed to be done? Because christ, it's faster to just set up a flask server with a custom /metrics endpoint than actually figure out how to do so using this framework.

从 Prometheus exporter 的调度文档和仓库中的第一个示例开始，该 issue 将其标识为基于定时器。添加一个基本示例，展示 Prometheus 抓取 exporter 时采集会同步开始；完成标准是该示例不再通过自己的定时器执行采集。