Add the KPI metrics handler (with no qualifying path) exactly once to each routing

[tjquinno]

Resolves #3240

The extended KPI metrics implementation in microprofile/server adds a handler in the chain that places a "deferrable" KPI metrics context in the request context. (Later, JerseySupport retrieves and updates the metrics context.)

Previously, the code that added the handler would see if the Application had an explicit setting for the path/context and, if so, would use that in adding the handler which added the metrics context to the request.

That was the problem--because the paths within that context would not match the routing for the handler, the KPI metrics context would (incorrectly) not be added to the request context.

The fix always adds this context-establishing handler to each routing so all requests will have the KPI metrics context set up and so all requests will update the metrics context (and the metrics it delegates to).

In fact, it was slightly inefficient to use the application path in the first place. In the rare (but possible) case of multiple JAX-RS apps, the old approach would have inserted the context-adding handler once for each application. This would not have caused the KPI metrics to be updated multiple times because the attempt to add the KPI context to the request context multiple times would have removed all the but last one, but we might as well avoid that inefficiency on each request.

To avoid this problem, the code makes sure to insert the context-adding handler only once to each named routing or the unnamed routing.

The PR also includes a test which configures the executor with 1 thread and blasts two concurrent requests to make sure the deferred metric is updated and will show a non-zero count.