OpenMetrics support for textfile collector

Currently prometheus_client.exposition.write_to_textfile doesn't support the OpenMetrics text format, which I'm addressing with this (private) hack:

from prometheus_client.openmetrics.exposition import (
    generate_latest as generate_latest_openmetrics,
)
from prometheus_client.registry import CollectorRegistry

def write_openmetrics_to_textfile(path: str, registry: CollectorRegistry) -> None:
    """
    Fork of `prometheus_client.exposition.write_to_textfile`, as the project
    didn't provide an OpenMetrics variant of this helper function.
    """
    with open(path, "wb") as fp:
        fp.write(generate_latest_openmetrics(registry))

Would your project be interested in a non-hacky (as is the case above) OpenMetrics support for textfile collector?

从 prometheus_client.exposition.write_to_textfile 入手，并将其行为与 prometheus_client.openmetrics.exposition.generate_latest 进行比较。确定 helper 应如何提供 OpenMetrics 输出，同时保留现有的文本文件 collector 行为。当受支持的 helper 为提供的 registry 写入有效的 OpenMetrics 文本，并且有项目相关测试覆盖时，即视为完成。