Add feature flag metric

[devknoll]

The detail field in performance.mark doesn't seem to be available in HTTP Archive, so this PR adds a custom metric that extracts a set of feature names according to the User Timing specification. This also includes support for the previous name, since some pages are using it.

Test websites:

• https://angular.io/
• https://angular.dev/
• https://svelte.dev/

[rviscomi]

FYI if you include this markdown in your PR description, the GH action will automatically test your changes on the provided URLs:

**Test websites**:

- https://example.com/

(I've updated the PR template so it should be clearer next time)

[devknoll]

Neat 👍 I edited and kicked it off so I could see :)

[github-actions]

Custom metrics for https://almanac.httparchive.org/en/2022/

WPT test run results: http://webpagetest.httparchive.org/results.php?test=240109_KZ_2

Custom metrics for https://angular.io/

WPT test run results: http://webpagetest.httparchive.org/results.php?test=240109_1V_3
Changed custom metrics values:

{
    "_aurora": {
        "ng_version": "17.0.0",
        "ng_img_user": null,
        "ng_priority_img_count": null,
        "nuxt_version": null,
        "nuxt_vue_version": null,
        "react_version": null,
        "svelte_version": null,
        "feature_flags": [
            "NgControlFlow"
        ]
    }
}

Custom metrics for https://angular.dev/

WPT test run results: http://webpagetest.httparchive.org/results.php?test=240109_08_4
Changed custom metrics values:

{
    "_aurora": {
        "ng_version": "17.0.0-rc.1",
        "ng_img_user": null,
        "ng_priority_img_count": null,
        "nuxt_version": null,
        "nuxt_vue_version": null,
        "react_version": null,
        "svelte_version": null,
        "feature_flags": [
            "NgHydration",
            "NgDefer",
            "NgAfterNextRender",
            "NgHttpTransferCache"
        ]
    }
}

Custom metrics for https://svelte.dev/

WPT test run results: http://webpagetest.httparchive.org/results.php?test=240109_CD_5
Changed custom metrics values:

{
    "_aurora": {
        "ng_version": null,
        "ng_img_user": null,
        "ng_priority_img_count": null,
        "nuxt_version": null,
        "nuxt_vue_version": null,
        "react_version": null,
        "svelte_version": "4",
        "feature_flags": []
    }
}

[rviscomi]

@pmeenan since WPT has support for custom marks and measures, is this something that could be useful upstream?

[rviscomi]

This LGTM. The WPT question shouldn't be blocking.