`Logger.MetadataProvider.multiplex` returns optional

Unless there is some reasoning I can't find on the repo or forums, this is a basic type discrepancy from [SLG-0001](https://github.com/apple/swift-log/blob/6f3fbace6629c8fcabcd56d211501e92e1919a94/Sources/Logging/Docs.docc/Proposals/SLG-0001-metadata-providers.md?plain=1#L161) and an instance is always provided anyways.

PR: https://github.com/apple/swift-log/pull/442

It is annoying to fallback to force unwrap (which looks spooky) or making an empty provider for coalescing:

```swift
.multiplex(myProviders) ?? .init({[:]})
```

---

Aside: I would also request some cleanup around `MetadataProvider.init` to take an `autoclosure`'d `Metadata`, rather than enforcing the closure style.

Start with Logger.MetadataProvider.multiplex and compare its declaration with SLG-0001-metadata-providers.md at the cited line. Review PR 442 and the surrounding API usage to identify any required updates; done means the return type matches the proposal without optional fallback, while the separate MetadataProvider initializer request remains out of scope.

Written by the indexing model from the issue text.