default Long serverSessionExpirationTimeNanos() {
        return null;
    }
Return the session expiration time, if any, otherwise null. The value is in
nanoseconds as per {@code System.nanoTime()} and is therefore only useful
when compared to such a value -- it's absolute value is meaningless. This
value may be non-null only on the server-side. It represents the time after
which, in the absence of re-authentication, the broker will close the session
if it receives a request unrelated to authentication. We store nanoseconds
here to avoid having to invoke the more expensive {@code milliseconds()} call
on the broker for every request

@return the session expiration time, if any, otherwise null
clients/src/main/java/org/apache/kafka/common/network/Authenticator.java