Make `Span` and `Metric` pull well-known props from `P` instead of direct fields

[KodrAus]

Currently, Span doesn't accept a SpanCtxt on construction because that should be part of the ambient context. This makes its representation incomplete, and puts the span context at a different level than the span name.

If we add SpanCtxt to Span and start including it in the Event's props when emitting it, we'll end up duplicating those properties. It's not technically an issue, it just looks a bit weird when you're using a non-deduplicating representation like fmt::Debug. It would be nice to work around this if we can, but I think that's the lesser issue than having Span not carry its own context.

[KodrAus]

An alternative design here is to make Span and Metric only accept Props, and add helper functions for pulling the well-known fields off them, but don't try specialize them on construction. With #374, it's much easier to create props with owned data in them, so I think this might be the better way to go.

[KodrAus]

Going with Option 2 here; Span and Metric will be changed to only accept Props, and use helper methods to retrieve their well-known values.