Add XML documentation for Android.Media.Metrics

The `Android.Media.Metrics` namespace contains **818** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

```powershell
rg -n -i --glob 'Android.Media.Metrics/**' --glob 'ns-Android.Media.Metrics.xml' 'to be added' docs\xml
```

## Update log

- 2026-09-10: Refreshed the placeholder scan from **794** to **818** (increased by **24**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start with the Android.Media.Metrics XML documentation files under docs\xml and run the provided rg scan to enumerate the 818 placeholders. Review each `To be added` entry in that namespace and replace it with documentation; the work is done when the scoped scan finds no remaining placeholders, excluding docs\xml\index.xml.

索引モデルが issue の本文から書いたものです。