Asserting OpenTelemetry data when using DistributedApplicationTestingBuilder

When using `DistributedApplicationTestingBuilder` in an integration test, because there is no dashboard process running, there is no OTLP endpoint hosted for the running resources to send OTel data to. We should consider having `DistributedApplicationTestingBuilder` host an OTLP endpoint and expose its data to allow assertion against it in a test, so that testing based on observing OTel data could be facilitated.