Have a way to add an async metadata provider

[arielb1]

Currently, the IMDS metadata provider is special, since it runs in the async task, and therefore if it is stuck, it is stuck in the background, and therefore it only breaks profiling rather than the entire application.

Consider having a way for users to specify a similar async metadata provider, e.g. with a

fn with_async_metadata_provider(self: &mut ProfilerBuilder, provider: impl FnOnce() -> impl Future<Result<(), impl Error>>)

(yes I know this type does not compile, consider some variant on it that does compile).