add complete documentation

Hey.

It would be nice if there was something like a write_to_stdout() function, or if write_to_textfile() would somehow allow writing to stdout.

For debugging that would seem to be much more helpful. Also, distros may have their own unified means of collecting the .prom files and thus even in production it may make sense to let a program simply write the metrics to stdout.

Cheers,
Chris.

PS: Using /dev/stdout doesn't work, as it tries to write it's tmpfile there.

首先定位 write_to_textfile() 并检查它如何处理 /dev/stdout 的情况。确定在保留现有文件输出的同时将指标直接写入 stdout 时的预期行为，然后验证两条输出路径都能按预期工作。issue 中未指定任何文件或测试。