4.x: Global instance handling change (and relevant changes to testing)

[tomas-langer]

Resolves #9186

• using ContextValue that uses Context as the "backing" store
• modified Metrics, ServiceRegistry, Config, and MapperManager to use ContextValue
• ContextValue either uses the test context, or global context; this is achieved through a context registered under a known qualifier, to make sure it all works even when running in child context

Introduction of new test annotations

• configuration setup using annotations for Helidon SE (through service registry)

• static configuration update (if value was not yet read, and is not provided by existing config sources)

• introduction of new JUnit5 annotation + extension (resets global instances)

• Fix security's ClassToInstanceStore (discovered an issue while copying some ideas to GlobalInstances)

• Add support for ExistingInstanceDescriptor to service registry (for testing purposes - bind an instance without a service descriptor)

• Testing JUnit extensions now extend the new extension, so all tests inherit the "nice" behavior of global instances SE testing now supports ServiceRegistry (and is ready for future setup using Helidon Inject)

[danielkec]

FAQ topic candidate

[romain-grecourt]

Do we event need this method ?
This looks like a hack that we aren't using, at least in this pull request.

One should be able to use this instead:

@TestConfig.Source
static ConfigSource config() {
    var config = TestConfigSource.create();
    onPseudoEvent(() -> config.set("foo", "bar"));
    return config;
}

[romain-grecourt]

We should provide another variant that takes a initial map.
E.g.

TestConfigSource.create(Map.of("foo", "bar"));

[romain-grecourt]

Do we event need this method ?
This looks like a hack that we aren't using, at least in this pull request.

One should be able to use this instead:

@TestConfig.Source
static ConfigSource config() {
    var config = TestConfigSource.create();
    onPseudoEvent(() -> config.set("foo", "bar"));
    return config;
}

[romain-grecourt]

Classifier used to register contexts as "singleton" capable.

[romain-grecourt]

ContextSingleton.CLASSIFIER

is better than

ContextSingleton.STATIC_CONTEXT_CLASSIFIER

At the very least we should not use "static", so either SINGLETON_CONTEXT_CLASSIFIER, CONTEXT_CLASSIFIER or just CLASSIFIER.