public Status progressed(ProgressToken progress) {
            if (state != State.RUNNING)
                throw new IllegalStateException("Current state must be RUNNING when updating progress");
            synchronized (progress) {
                return new Status(startedAt, null, progress.serializeToString(), state, null);
            }
        }
Returns a copy of this with the given progress.
clustercontroller-reindexer/src/main/java/ai/vespa/reindexing/Reindexing.java