configuration migration of telemetry configuration loses service_name

[garypen]

If an older router configuration looks like this (in a file router.yaml:

telemetry:
  tracing:
    common:
      service_name: router
    trace_config:
      attributes:
        env: dev

executing: cargo run -- config upgrade router.yaml to update the configuration will yield:

telemetry:
  exporters:
    tracing:
      common:
        resource:
          env: dev

Note: service_name is lost.

We believe that: "the move of trace_config to common is zapping the existing common content.".