Thanks for the quick reply and PR @felixbarny. These are jsonRPC calls being handled by our custom library so the meaningful values will be in the request body. However I think your change is still useful for other services.
We are actually using the opentracing-api instead of apm-agent-api directly since there are shims available from both Elastic APM and OpenTelemetry for the OpenTracing Tracer.
We are forced to use a mixed setup because we had scaling issues using the Elastic APM Java Agent on our very high rps services (40,000+ rps). Even with a low sample rate the high level transactions are still reported to the APM server and that slowed things down significantly (see https://github.com/elastic/apm/issues/104 and https://github.com/elastic/apm/issues/151).
Switching to using the OTEL or Jaeger agent and exporting to APM server's Jaeger endpoint works, but it is a hacky solution. I'm not sure if things have changed recently, but if it would be possible for the Java agent to only send sampled transaction information instead of all transactions that would make thing scale much easier.