mockotlpserver: compare JSON serialization form to OTel Java's "Logging OTLP JSON exporter"

https://opentelemetry.io/docs/languages/java/configuration/#logging-otlp-json-exporter

In mockotlpserver, some serialization decisions were made the object form of an OTLP http/json service request. That form is used in our tests, and also in some OTel JS contrib repo tests via the `runTestFixture` facility we added in the contrib-test-utils package. I'm curious if/how this form differs from the serialization form used by the OTel Java otlp-json exporter -- an experimental exporter for dev/debugging. If there are differences, it might be good to align.

This is low prio.