In https://static.javadoc.io/org.springframework.cloud/spring-cloud-sleuth-core/1.2.0.RELEASE/org/springframework/cloud/sleuth/Span.html#SPAN_ID_NAME there's a constant for X-B3-SpanId but that constant does not appear to be in spring-cloud-sleuth-core-2.1.0.RELEASE.jar
Is there a new place for that constant?
With Sleuth 2.0 we've migrated to Brave (https://github.com/apache/incubator-zipkin-brave) as the new Tracer library. The new place for that constant is here https://github.com/apache/incubator-zipkin-brave/blob/v5.6.4/brave/src/main/java/brave/propagation/B3Propagation.java but mind you that it's a package scope value. Which means that you have to define your own constant. You can however retrieve it via the https://github.com/apache/incubator-zipkin-brave/blob/9fc4a977aba15c5af7a4bacddaf4d7f7267a663a/brave/src/main/java/brave/propagation/B3Propagation.java#L81 keys method.