Jaeger/Zipkin/Opencensus exporters should populate service_name from Span Resource

[lzchen]

Currently, the three exporters that require service_name collect them all in different ways.

Jaeger -> after this pr, will be populated by global tracer_provider -> resource.
Opencensus -> passes in constructor
Zipkin -> Populated through global tracer_provider -> resource code

Following from @owais 's comment, users can instantiate multiple TracerProviders, which could possibly have different Resource attached to them, and therefore the service_name s could be different.

Dot net SDK has a reference to the creating tracer_provider in their exporters (which is weird), and Java SDK simply uses the currently exporting Span 's resource to populate service_name. I propose that we do something similar to Java for our exporters.

[dmarar]

In a batch will all the spans have the same resource?
As per the spec https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/resource/sdk.md

When used with distributed tracing, a resource can be associated with the TracerProvider when the TracerProvider is created. That association cannot be changed later. When associated with a TracerProvider, all Spans produced by any Tracer from the provider MUST be associated with this Resource.

Assuming that all spans from a tracer will have same resource we could do something similar to
#1607 (comment)

just a thought.

[owais]

@dmarar Yes, this makes a lot of sense to me. Span translator should copy resource attributes to process tags (jaeger) and span tags (zipkin).

[srikanthccv]

I am not sure if you can just take the 0th span and make it's resource as whole batch resource. I haven't closely looked at the span processor code so I don't know yet if it is possible for batch to have spans originating from different resources. Would it be more better if we group them by their resource and make proto.PostSpansRequest/thrift.Batch export for each of them?

[lzchen]

From SIG meeting 03/11/2021 -> We will be restricting users to have a new instance of spanprocessor for each TracerProvider they create, so we can go with taking the 0th span to populate service_name for whole batch.

[lzchen]

Work finished for Zipkin and Jaeger.

[codeboten]

Created a separate issue specifically for the opencensus exporter