did you consider extending AbstractHttpServerUsingTest instead of introducing this class?

AbstractHttpServerUsingTest carries a load of existing tests with it. I just want to add one new test that is independent of AbstractHttpServerUsingTest's existing tests.

The WebClient setup in this class is copied from private code in this lib. If I am allowed to modify that code so that I can access then I can refactor or remove this new class.

I think you are mixing AbstractHttpServerUsingTest with AbstractHttpServerTest. AbstractHttpServerUsingTest contains zero tests.

The reason I need this new class is because I need a WebClient instance. AbstractHttpServerUsingTest does not create a WebClient.

HttpServerInstrumentationExtension creates a WebClient but it needs an InstrumentationTestRunner.

These are all major rabbit holes.

I can get rid of this class and just create my own WebClient in my test class. I just didn't want to write this in Scala but I will if this Java class is a problem for you.

Typically you'd extend the AbstractHttpServerUsingTest and in the test register the extension with

@RegisterExtension
static final InstrumentationExtension testing = HttpServerInstrumentationExtension.forAgent();

Then you can use the WebClient in your test. You can also use testing.runWithSpan(...) to create spans and testing.waitAndAssertTraces(...) to verify that the spans have the correct structure instead of trying to encode trace and span info in the http response. I'd suggest you give it a try, it will be much nicer than what you currently have.

ok - I'll abandon this

I don't think it is healthy to have such a high bar for tests.