Support external services in the service map

[dlvenable]

Is your feature request related to a problem? Please describe.

Some services call external services which are not annotated. Data Prepper users still want to see these external services in the service map.

Describe the solution you'd like

Include external service calls in the service map. This issue still needs additional analysis for the solution.

Additional context

Related issue in the OpenSearch Observability plugin: opensearch-project/dashboards-observability#90

Also, see #628 for a related Data Prepper issue which is somewhat the opposite problem.

[dlvenable]

The problem for this issue is that we have spans for an un-instrumented service. Data Prepper should be able to output a service map document which has the external un-instrumented service.

[dlvenable]

Utilizing the peer.service might be appropriate here.

https://opentelemetry.io/docs/specs/otel/trace/semantic_conventions/span-general/#general-remote-service-attributes

We've seen some spans that have the following where an external system is used and not included in the service map.

"span.attributes.peer@service": "externalSystem"