In certain deployments, it is not possible to know the service-cluster apriori (e.g., kubernetes pods). In such cases, the service to which the pod belongs do, has to be resolved dynamically, and the span data generated by the tracing subsystem needs to be updated with this info.

High level Implementation proposal:
We need the StreamFilterCallback to expose a span interface, that would allow the filters to obtain a (writable) reference to the active span object in the active stream and be able to modify them, before Envoy finalizes the span.

cc @goaway @mattklein123