SpanAttributes as a type

[ktoso]

Since the spec suggests those should keep order, just a plain old dict is not going serve this well:

Attributes SHOULD preserve the order in which they're set. Setting an attribute with the same key as an existing attribute SHOULD overwrite the existing attribute's value.

This basically means a SortedDict which we don't have;

It's a SHOULD so we can move forward without it for now but we want to be in a place where we can swap the impl without breaking anyone to conform to this requirement.

So, let's:

struct SpanAttributes { 
    subscript(_ name: String) -> SpanAttribute? 
    func forEachAttribute(_ callback: (String, SpanAttribute) -> ())
}

and for now back it with a private [String:SpanAttribute] which we can easily replace eventually