No API for for async context propagation in Client API

[pdudits]

Currently the client offers no API for capturing calling thread context when invocations are turned into async on RX ones. ClientRequestFilters are first executed on executing thread.

Proposal:

New component that can be registered into ClientBuilder with roughly following API:

interface AsyncContextProvider {
   void initAsyncContext(RequestContext asyncContext);
}

These providers would be looked up and invoked by AsyncInvoker or before turning an invocation into RxInvoker, to allow for capturing additional calling thread context into request context.

[jansupol]

• There is Invocation.Builder#property to set properties to be accessible on the ClientRequestFiter

• RxInvoker has an RxProvider spi that basically makes it hard to make sure the AsyncContextProvider would run in a non-executor thread. Mostly, the RxProvider runs the SyncInvoker in a thread given by the executor service, so it is late already. An option is to run the AsyncContextProvider as soon as the async()/rx() is executed. But the request may never be actually invoked, so it does not sound right to execute that at the point.

• For AsyncInvoker this can be done

• For default RxInvoker this can be done, too.

@pdudits What do you think the right solution for RxInvoker would be?

[jansupol]

I have created #4301, which supersedes this requirement. Does it look good to you?

[pdudits]

Oh, forgot to react last time, the PR looks interesting, I will try it out.

[jansupol]

@pdudits Thanks, I am open to suggestions for improvements.