Add a CI to verify that usage telemetry JSON conforms to the Checkpoint server spec

We've broken usage telemetry because we changed the schema of the json object and forgot to update the checkpoint server.

Let's create a CI test that validates that usage telemetry metric emission works with every PR. Preferably we would start a new server from the build image, but it's fine if the test is done against the running server if that is simpler.

We can also have an always running a dev server in a container next to the official one.