Zipkin exporter: default service name

[MrAlias]

Currently in the exporter we default to "" if the resource doesn't contain a service name:

opentelemetry-go/exporters/trace/zipkin/model.go

Lines 43 to 52 in e9aaa04

	func getServiceName(attrs []attribute.KeyValue) string {
	for _, kv := range attrs {
	if kv.Key == semconv.ServiceNameKey {
	return kv.Value.AsString()
	}
	}
	// Resource holds a default value so this might not be reach.
	return ""
	}

The specification states:

If no service.name is contained in a Span's Resource, it MUST be populated from the default Resource.

While it is a good chance that the passed resource will be a merge with the default, it needs to explicitly be set to the default in the exporter.

[lastchiliarch]

I'd like to take this.
Seems like we should use resource.Default() to get the default resource and then try to get the semconv.ServiceNameKey as the default service name.

[MrAlias]

Seems like we should use resource.Default() to get the default resource and then try to get the semconv.ServiceNameKey as the default service name.

Correct 👍

[MrAlias]

Resolved by #1786