Support IO streams in MetricWriter

[msto]

Summary

MetricWriter currently only accepts a file path, requiring users to write to actual files. It would be more flexible to accept an IO or TextIOWrapper directly, enabling writing to any IO source.

Context

From #6 (comment) (@clintval):

It'd be nice if this accepted an IO or io.TextIOWrapper and a classmethod def from_path() handled the opening of the file. That way, users could read metrics from any IO source without having to mock them up into files.

Suggested Solution

Refactor MetricWriter.__init__ to accept an IO handle directly, and add a from_path() classmethod for file-based usage. Use an ownership flag to determine whether the writer should close the handle on exit.

1. Introduce a new _owns_handle: bool attribute on MetricWriter
2. __init__ sets _owns_handle = False by default, so when working with a passed IO object the caller manages the lifecycle of the handle
3. from_path() opens the file and sets _owns_handle = True, so the handle is closed on exit of the writer

MetricWriter should remain a context manager in both cases