Add a trace option to the Metal Backend

This works by setting DAWN_TRACE_FILE_BASE=foo which results
in a trace foo-yyyy-mm-dd-hh-mm-ss-count.gputrace being written.
You can also set DAWN_TRACE_DEVICE_FILTER to have traces only written
when the device's label contains the filter string.

The trace runs from device creation until device destroy so
no extra API needed.

Ultimately it would be nice to make this work on all backends
with the same environment variables but this was easy to put
in and is useful for devs on Metal, especially in dawn.node
with compute only captures.

To use you need somethnng like

```
METAL_CAPTURE_ENABLE=1 DAWN_TRACE_FILE_BASE=foo prog-that-uses-dawn-metal
```

Which will write out something like `foo-2025-02-12T12-05-07-c000.gputrace`

Bug: 396072565
Change-Id: I67271f3fff8ac18f56e48a4960f9ef493f77bfcf
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/226174
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Gregg Tavares <gman@chromium.org>