gcp-resources documentation doesn't align with the GCP Resource Detector

[bcarter97]

The GCP Resource Detector mentions - and contains docs for - k8s Resource attributes. However, none of them aside from k8s.cluser.name appear to be instrumented:

opentelemetry-java-contrib/gcp-resources/src/main/java/io/opentelemetry/contrib/gcp/resource/IncubatingAttributes.java

Lines 57 to 58 in d06d470

	public static final AttributeKey<String> K8S_CLUSTER_NAME =
	AttributeKey.stringKey("k8s.cluster.name");

It looks like they exist in the GCP Resource Translator, so they're only missing in the Otel side. I noticed they're in the IncubatingAttributes so perhaps they're still in development?