Access all attributes from ReadableSpan

[jack-berg]

Currently if you're writing a span processor and want to read all of a span's attributes, you need to first get an immutable copy of that span via ReadableSpan#toSpanData().

Oddly, ReadableSpan has a method for accessing a single attribute if you know the key ahead of time, but not all attributes. This seems to be a simple oversight. The specification states the following on Readable Span:

A function receiving this as argument MUST be able to access all information that was added to the span, as listed in the API spec for Span.

Given that accessors are available for many parts of the span, its odd that attributes are not accessible in the same way. Clearly the trick of converting toSpanData() first to access all fields was not the intended usage patter, else why have any direct accessors on ReadableSpan in the first place?

I propose we add Attributes getAttributes to ReadableSpan.