@Override public MockResponse peek() {
        MockResponse maybeFailure = failureQueue.peek();
        if (maybeFailure != null) return maybeFailure.clone();
        return new MockResponse().setSocketPolicy(KEEP_OPEN);
      }
Starts up a local Zipkin server, listening for http requests on {@link #httpUrl}.

<p>This can be used to test instrumentation. For example, you can POST spans directly to this
server.

<p>See http://openzipkin.github.io/zipkin-api/#/
zipkin-junit5/src/main/java/zipkin2/junit5/ZipkinExtension.java