Remove "FollowsFrom" relationships

[iredelmeier]

Reference is not defined in the specs. What does a follow from mean?

Originally posted by @bogdandrutu in #190

Note that this would affect FollowsFrom-based relationships in the OpenTracing bridge.

[jmacd]

"Reference" may be a misleading term. I thought we called these relationships.

I think it will be a set-back if we remove FollowsFrom.

[iredelmeier]

@jmacd that's my understanding too re: "relationships" instead of "references". The first line is a quote since I created this issue from a discussion in #190.

[rghetia]

Use attribute "span.relation" = "childOf" or "followsFrom" in Link.

[lizthegrey]

I'm confused though, doesn't Link lack the relation or RelationshipType field?

Is

Relation{
	SpanContext:      sc,
	RelationshipType: FollowsFromRelationship,
}

being replaced with:

Link {
	SpanContext: sc,
	Attributes []core.KeyValue
}

which lacks the RelationshipType field?

[lizthegrey]

and how does this relate to #225

[jmacd]

I feel we still have no substitute for "follows from", see here open-telemetry/opentelemetry-specification#337 (comment)

[lizthegrey]

I chose to do #272 in order to implement Links at span creation time, rather than trying to munge FollowsFrom into a Link etc

[rghetia]

fixed by #451