[Bug]: non-root spans with FollowsFrom reference incorrectly represent as trace in collector metrics

[ChenX1993]

What happened?

We noticed that non-root spans with FollowsFrom reference are showing as traces in collector metrics, e.g. traces.received, traces.rejected metrics.

The reason is that in collector metrics, ParentSpanID() is used to determine if a span is a root span, e.g. below

jaeger/cmd/collector/app/metrics.go

Lines 233 to 237 in 3d67ac8

	m.countSpansByServiceName(serviceName, span.Flags.IsDebug())
	if span.ParentSpanID() == 0 {
	m.countTracesByServiceName(serviceName, span.Flags.IsDebug(), span.
	GetSamplerType())
	}

And the current implementation of ParentSpanID() only considers ChildOf relationship. So if a span has only FollowsFrom reference, the ParentSpanID() doesn't consider that and returns 0.

jaeger/model/span.go

Lines 99 to 109 in 506adaf

	// ParentSpanID returns ID of a parent span if it exists.
	// It searches for the first child-of reference pointing to the same trace ID.
	func (s *Span) ParentSpanID() SpanID {
	for i := range s.References {
	ref := &s.References[i]
	if ref.TraceID == s.TraceID && ref.RefType == ChildOf {
	return ref.SpanID
	}
	}
	return SpanID(0)
	}

Not sure any particular reason that FollowsFrom is not considered in the current implementation of ParentSpanID().

The proposal is to either 1) fix in collector metric to replace the ParentSpanID(), 2) fix in ParentSpanID().
Prefer 2) because we use this method a lot to determine a span is a root span internally, but changing its behavior might break other places in jaeger OSS.

Steps to reproduce

Create spans with FollowsFrom relationship, and check the collector trace metrics.

Expected behavior

non-root spans with FollowsFrom reference should not show as trace in collector metrics.

Relevant log output

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Screenshot

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Additional context

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Jaeger backend version

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SDK

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Pipeline

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Stogage backend

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Operating system

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Deployment model

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Deployment configs

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[kubarydz]

Would chaning ParentSpanID() behaviour in 2) way also resolve this (rather old one) TODO?

jaeger/model/adjuster/clockskew.go

Lines 117 to 126 in 506adaf

	// finds all spans that have no parent, i.e. where parentID is either 0
	// or points to an ID for which there is no span.
	func (a *clockSkewAdjuster) buildSubGraphs() {
	a.roots = make(map[model.SpanID]*node)
	for _, n := range a.spans {
	// TODO handle FOLLOWS_FROM references
	if n.span.ParentSpanID() == 0 {
	a.roots[n.span.SpanID] = n
	continue
	}

[ChenX1993]

I think so.