WO-1582 Refactor tracing span implementations

[jmorrell-cloudflare]

Preparatory work for tracing.getActiveSpan

We've ended up with branching logic within SpanImpl. Mostly now it's whether the span is observed or not:

if (!builder.isObserved()) {
  return;
}

In the next PR I'll be introducing the Invocation span, which will have it's own set of logic. Most notably that we should ignore .end() as a no-op since that is owned by the platform. This will have more branching logic.

export default function fetch(...) {
  // get the root span for the invocation
  const span = ctx.tracing.getActiveSpan();

  // this is cool, and we want to encourage thsi
  span.setAttributes({ "user.id": 123 });

  // but if a user does this, we need to ignore it
  span.end(); 
}

And we're likely going to end up with a 4th case, which otel calls a NonRecordingSpan where a span is created representing the remote span that is also a no-op, but does allow children to be created.

const span = tracing.trace.wrapSpanContext({
  traceId: '0123456789abcdef0123456789abcdef',
  spanId: '2222222222222222',
  traceFlags: 1,
});

With that in mind, this moves SpanImpl into the SpanState interface, where we can provide multiple implementations. For now UserSpanState and NoopSpanState.

If this winds up being easier to review with the next change, I'm happy to combine them.

[ask-bonk]

LGTM

github run