Log with level ERROR when try to get feature value first time

[roman-struchev]

Context
We use feature hub to manage feature toggles

Also, we have 2 additional point

• not all feature toggles exist in feature hub, the responsible person often creates it when feature toggle is needed
• we have several feature toggle sources with priority, for example, feature hub, application properties, environment, etc

Issue

featurehub-java-sdk/client-java-core/src/main/java/io/featurehub/client/ClientFeatureRepository.java

Lines 247 to 259 in f6fd04b

	public FeatureState getFeatureState(String key) {
	return features.computeIfAbsent(
	key,
	key1 -> {
	if (hasReceivedInitialState) {
	log.error(
	"FeatureHub error: application requesting use of invalid key after initialization: `{}`",
	key1);
	}
	return new FeatureStateBase(null, this, key);
	});
	}

When we try to get feature value in java (io.featurehub.client.ClientFeatureRepository#getFeatureState) first time, then we have such log for our applications FeatureHub error: application requesting use of invalid key after initialization: XXX.

This happens only for the first request for the feature toggle (not existing in feature hub), after instance of application was restarted (because of java.util.Map#computeIfAbsent used)

So in monitoring of our applications we have error cases by logs

Fix
Idea how to fix #34