AbstractCouchbaseRequest doesn't lend itself to RxJavaHook

Many libraries, and people, depend on the RxJava instrumentation beyond the plethora of (very) useful operators; one such library is Spring Cloud Sleuth. Turns out, when using Couchbase Java client, that uses RxJava 1.x (master is using 1.6.0-SNAPSHOT as of this writing), Sleuth loses track of the query. After some back and forth with the developers, and based on my own investigation, the problem appears to be here.

Original ticket I opened against Sleuth: https://github.com/spring-cloud/spring-cloud-sleuth/issues/914

Spin-off ticket in Zipkins: https://github.com/openzipkin/brave/issues/664

RxJava ticket: https://github.com/ReactiveX/RxJava/issues/5949

RxJava 1.x is end-of-life and no further development is being done. Thus, Sleuth or RxJava are reluctant in making any changes to their code. Couchbase client should upgrade to RxJava 2, or even 3, or Project Reactor. For the short term, I'm looking for a fix along the lines suggested in https://github.com/ReactiveX/RxJava/issues/5949.

See attachment from Kibana showing the lack of trace id and span id.