default FetchState transitionTo(FetchState newState) {
            if (validTransitions().contains(newState)) {
                return newState;
            } else {
                return this;
            }
        }
The fetch state of a partition. This class is used to determine valid state transitions and expose the some of
the behavior of the current fetch state. Actual state variables are stored in the {@link TopicPartitionState}.
clients/src/main/java/org/apache/kafka/clients/consumer/internals/SubscriptionState.java