Prevent exception in HttpServer exporter when using custom root metrics path

[Donnerbart]

The behavior of HttpServer.createContext() was changed between Java 11 and 21. In the current JDKs the server doesn't allow to register multiple contexts with the same path. So if you configure

HTTPServer.builder()
            .port(0)
            .registry(registry)
            .metricsHandlerPath("/")
            .buildAndStart()

you get the following exception:

java.lang.IllegalArgumentException: cannot add context to list
	at jdk.httpserver/sun.net.httpserver.ContextList.add(ContextList.java:37)
	at jdk.httpserver/sun.net.httpserver.ServerImpl.createContext(ServerImpl.java:306)
	at jdk.httpserver/sun.net.httpserver.HttpServerImpl.createContext(HttpServerImpl.java:69)
	at jdk.httpserver/sun.net.httpserver.HttpServerImpl.createContext(HttpServerImpl.java:34)
	at io.prometheus.metrics.exporter.httpserver.HTTPServer.registerHandler(HTTPServer.java:111)

This PR fixes the issue by skipping the default handler registration in case the metrics are configured for the root path.

I also improved the unit test so we can properly assert expectedStatusCode and expectedBody. With this improvement we can be sure that an endpoint actually returns the expected handler content, not just a matching status code.