OpenTelemetry active message consumption spans are incorrectly classified as transactions

APM Server converts OpenTelemetry spans to either transactions or spans, depending on several criteria:
- root OTel spans become Elastic APM transactions
- server and consumer OTel spans become Elastic APM transactions
- all other OTel spans become Elastic APM spans

The issue with converting all consumer OTel spans to Elastic APM transactions is that not all consumer spans represent service entrypoints. For example, a message may be actively consumed (e.g. with active polling) within another OTel span. In this case, the outer span should become an Elastic APM transaction, and the message consumption span should become an Elastic APM span.

We require additional information to be recorded in OTLP for this: see https://github.com/open-telemetry/oteps/pull/182