Tracing not propagated when using SE and Jersey client

[tomas-langer]

Helidon 1.1.2

When I use jersey reactive client, tracing is not automatically propagated and I need to send SpanContext as request property:

Invocation.Builder requestBuilder = webTarget.request();

        // propagate security if defined
        request.context()
                .get(SecurityContext.class)
                .ifPresent(ctx -> requestBuilder.property(ClientSecurityFeature.PROPERTY_CONTEXT, ctx));

        // propagate tracing
        requestBuilder.property(ClientTracingFilter.CURRENT_SPAN_CONTEXT_PROPERTY_NAME, request.spanContext());

        // and reactive jersey client call
        requestBuilder.rx()
                .get(String.class)
                .thenAccept(response::send)
                .exceptionally(throwable -> {
                    // process exception
                    response.status(Http.Status.INTERNAL_SERVER_ERROR_500);
                    response.send("Failed with: " + throwable);
                    return null;
                });

[tomas-langer]

Related to eclipse-ee4j/jersey#4227

[tomas-langer]

This problem is caused by the usage of rx(). This uses an executor service that cannot be easily overridden.
The PR for Jersey was an attempt to fix a behavior when explicit executor sevice (configured in client builder) is ignored and an executor service provider (registered using method register) is used instead.
We must create a Helidon client feature that would autoregister an executor service provider, so we can wrap the executor service with Contexts.wrap, as otherwise we cannot propagate context.

[tomas-langer]

Fix is based on automatic feature that configures an ExecutorServiceProvider with the Jersey client.
Behavior may be overridden via JaxRsClient.defaultExecutor to use an explicit executor service, or via JaxRsClient.configureDefaults(Config) to use a configuration based approach.