…with APM server

Env var is ELASTIC_APM_DISABLE_SEND. Default is false. This feature is
useful to use the APM agent for async context tracking, distributed
tracing context propagation, and/or log enrichment with trace.id. When
disableSend=true, attempts are made to limit unnecessary processing
(like calculating metrics, encoding stacktraces, etc.).

This is the same as the "disable_send" option in some of the other APM agents.

That should suffice to give debugging hint why data isn't being sent and
limit log.trace noise sufficiently.