@InterfaceStability.Unstable public interface ClosableFunction
close() to free resources used during the execution of the function, clean up state etc.
Order of finalization:
ClosableFunctions are closed in the reverse topological order of operators in the
StreamGraph. For any two operators A and B in the graph, if operator B consumes results
from operator A, then operator B is guaranteed to be closed before operator A.
default void close()