Let users create ReadOnlySpan instances from outside the SDK

[punya]

Problem Statement

Instead of exposing the mutable SpanSnapshot struct, the SDK now uses the ReadOnlySpan interface. This interface has an un-exported func, which prevents anybody outside the SDK from creating new implementations.

Creating new instances of ReadOnlySpan is valuable:

• when writing unit tests for an exporter
• when using an exporter without an SDK (e.g. in the collector)

Proposed Solution

type SpanFields {
  // same as SpanSnapshot fields from before
}

func NewReadOnlySpan(f SpanFields) ReadOnlySpan {
  // copy the fields to a spanSnapshot
}

This way,

• ReadOnlySpan remains immutable
• SpanSnapshot's fields remain unexported, so people can't cast their way out of the immutability
• API remains ergonomic if new fields are added to SpanFields (because they will be zero-initialized)

Alternatives

• Each user creates a copy of ReadOnlySpan and SpanSnapshot, and creates a private API to pass that in to their exporter (for example)
• Have NewReadOnlySpan take a bunch of WithFoo(...) args. This is a lot more code to write in the SDK, and seems unnecessary.
• [added] Users embed ReadOnlySpan in a struct and override any relevant methods (for example)

[punya]

For completeness, I should mention that we can embed the ReadOnlySnapshot interface in a struct as a way to implement the interface. This may lower the priority of this request, but I think it's still worth pursuing.

(Thanks to @bogdandrutu for suggesting this approach.)

[darwin67]

We have some reasons for wanting to implement our own version of ReadOnlySpan, where the traces doesn't map properly to a typical request/response, and a function call (stack) kind of flow.

Like @punya mentioned, I'm working around it by embedding the interface into our struct, but ideally I rather prefer not doing that and just outright implement the interface methods.