[APM] Service map can cause OOM in elasticsearch

[neptunian]

Following up from #186417

When testing the service map under the maximum conditions of 1k trace ids with each trace having ~500 spans, the scripted metric aggregation can cause an OOM in elasticsearch depending on the memory available. Looking at the elasticsearch heapdump, I suspect this is due to the # of hash maps and other data structures being created simultaneously, where data can be duplicated and exist at the same time within the reduce phase. This issue did not happen when disabling parallel async requests and having them sync, when calling fetch_service_paths_from_trace_ids. Further investigation needed.

[neptunian]

@crespocarlos did some investigation #187445

[elasticmachine]

Pinging @elastic/obs-ux-infra_services-team (Team:obs-ux-infra_services)

[botelastic]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.