Record links with empty span context

[MrAlias]

Links with invalid span context are now recommended to be recorded by the specification if they have non-empty attributes or trace state.

Implementations SHOULD record links containing SpanContext with empty TraceId or SpanId (all zeros) as long as either the attribute set or TraceState is non-empty.

Our implementation does not currently follow this behavior:

opentelemetry-go/sdk/trace/span.go

Lines 632 to 653 in 0fc35e0

	func (s *recordingSpan) AddLink(link trace.Link) {
	if !s.IsRecording() || !link.SpanContext.IsValid() {
	return
	}
	l := Link{SpanContext: link.SpanContext, Attributes: link.Attributes}
	// Discard attributes over limit.
	limit := s.tracer.provider.spanLimits.AttributePerLinkCountLimit
	if limit == 0 {
	// Drop all attributes.
	l.DroppedAttributeCount = len(l.Attributes)
	l.Attributes = nil
	} else if limit > 0 && len(l.Attributes) > limit {
	l.DroppedAttributeCount = len(l.Attributes) - limit
	l.Attributes = l.Attributes[:limit]
	}
	s.mu.Lock()
	s.links.add(l)
	s.mu.Unlock()
	}

The implementation needs to be updated and tests added to prevent regression.

[amanakin]

Hi! I'd like to take that one