[BUG] multi-node gRPC tests are flaky

[rursprung]

What is the bug?
see this description from @nibix:
#6310 (comment)

In the test framework, we have several nodes running in a single JVM. For an incoming GRPC request, the filter picks a backend registry instance from GuiceHolder. However, as this is a singleton, there is a 2:1 chance that it gets an instance from a wrong node. This will lead to the observed issue.

the problematic code is this here, which even includes a TODO:

security/src/main/java/org/opensearch/security/OpenSearchSecurityPlugin.java

Lines 2803 to 2817 in a7f4027

	// below is a hack because it seems not possible to access RepositoriesService from a non guice class
	// the way of how deguice is organized is really a mess - hope this can be fixed in later versions
	// TODO check if this could be removed
	@Override
	public Collection<Class<? extends LifecycleComponent>> getGuiceServiceClasses() {
	if (client || disabled || SSLConfig.isSslOnlyMode()) {
	return Collections.emptyList();
	}
	final List<Class<? extends LifecycleComponent>> services = new ArrayList<>(1);
	services.add(GuiceHolder.class);
	return services;
	}

How can one reproduce the bug?
switch a gRPC test to DEFAULT rather than SINGLENODE cluster and run it several times. it'll be flaky.

What is the expected behavior?
no flaky tests

What is your host/environment?
tested on main with OpenSearch 3.9.0-SNAPSHOT

Do you have any screenshots?
n/a

Do you have any additional context?
n/a

[cwperks]

ty for filing this issue @rursprung ! Everything we can do to improve CI stability is good.