Adding custom items to service-map traces

Is your feature request related to a problem? Please describe.
Traces coming from the otel-trace-pipeline and forwarded to the service-map-pipeline don't propagate some items. It could be a great improvement if custom items could be added in the service-map-pipeline.

Describe the solution you'd like
The trace I recieve at the moment from the service-map-pipeline is the following:

  {
    "_index" : "otel-v1-apm-service-map",
    "_type" : "_doc",
    "_id" : "4tvYCIifyVbGYfH/9iaP/g==",
    "_score" : 1.0,
    "_source" : {
      "serviceName" : "order",
      "kind" : "SPAN_KIND_INTERNAL",
      "destination" : {
        "resource" : "prepareOrderManifest",
        "domain" : "inventory"
      },
      "target" : null,
      "traceGroupName" : "createOrder",
      "hashId" : "4tvYCIifyVbGYfH/9iaP/g=="
    }
  }

This trace should be manipulated such that a "tenant" item is added. "tenant" is already available as a resource attribute within the trace in the otel-trace-pipeline :

  {
    "_index" : "otel-v1-apm-service-map",
    "_type" : "_doc",
    "_id" : "4tvYCIifyVbGYfH/9iaP/g==",
    "_score" : 1.0,
    "_source" : {
      "serviceName" : "order",
      "kind" : "SPAN_KIND_INTERNAL",
      "destination" : {
        "resource" : "prepareOrderManifest",
        "domain" : "inventory"
      },
      "target" : null,
      "traceGroupName" : "createOrder",
      "hashId" : "4tvYCIifyVbGYfH/9iaP/g==",
      "tenant" : "test1"
    }
  }

Additional context
Add any other context or screenshots about the feature request here.

Start by tracing how data moves from the otel-trace-pipeline into the service-map-pipeline and where resource attributes are selected for the service-map output. Use the supplied examples to determine how a custom item such as tenant should be configured or propagated, and consider the work complete when the resulting service-map trace includes it without losing existing fields.