Test telemetry from the Collector

[jpkrohling]

As part of #10490, I faced difficulties in testing the telemetry that was generated to ensure the change was doing what's supposed to do. I did some manual tests, but unit tests would be better here.

One possible alternative: implement a mock OTLP server (receiver), like https://github.com/open-telemetry/opentelemetry-collector/blob/main/exporter/otlpexporter/otlp_test.go

Another alternative: align with the regular testing helpers from the Go SDK. This might involve changes to the config spec, and not all SDKs might have in-memory exporters for all signals.