Define resource mapping for Jaeger export

[mwear]

What are you trying to achieve?

Mapping Resources to Jaeger is not straightforward due to the multiple tracer provider use case, see: https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/trace/api.md#tracerprovider. Ideally they would map to Jaeger process tags, however, since two tracer providers can have separate resources, but share the same exporter, an
exporter has to assume that spans are coming from multiple resources. I can see a couple of ways to handle this:

Post per resource: An exporter can group spans by resource, make separate posts for each resource, and map the resource to process tags.

Merge Resource Attributes with Span Attributes: Avoid the multiple resource issue by merging resource attributes with span attributes. This would have to be done for each span.

We could consider re-evaluating the multiple tracer provider use case. It leads to a number of other, similar complications.

Additional context.

Both Python and Go have solved this by merging Resource attributes with Span attributes. See:

• https://github.com/open-telemetry/opentelemetry-python/blob/master/exporter/opentelemetry-exporter-jaeger/src/opentelemetry/exporter/jaeger/__init__.py#L200
• https://github.com/open-telemetry/opentelemetry-go/blob/master/exporters/trace/jaeger/jaeger.go#L224-L230

We should decide how to handle this and document the decision in the jaeger spec

[mwear]

I wanted to add that this is a consequence of design decisions described in this comment: #508 (comment).

[mwear]

We discussed this during the spec SIG on Tuesday and decided that Resources should be exported as Jaeger process tags. This is what the collector does. See: https://github.com/open-telemetry/opentelemetry-collector/blob/cadd49875b3bcbc6ca555b66c7d09b6d3a78bff0/translator/trace/jaeger/traces_to_jaegerproto.go#L107-L133