What's the purpose of having a separate FlushGuard?

[PoignardAzur]

Having ChromeLayerBuilder return both a layer and a guard object is inconvenient. It means that would otherwise be encapsulated code creating a subscriber with a bunch of layers now needs to return that guard to the caller, even though the caller doesn't want to know anything about the tracing implementation. (Or mem::forget the guard.)

Is there any reason the guard couldn't be stored in the ChomeLayer directly? If so, that reason should probably be included in the documentation.