This looks unnecessary, right?

I agree. Removed in 6e73409

Nit: maybe you can remove the dots.

Should this return the future here? probeTransaction returns CompletableFuture.completedFuture(null), but seems to be never used. I think that this could be reworked as follows to prevent duplicating logic:

if (transaction == null) {
    transaction = transactionWriter.beginTxn();
}
if (this.probeRequested(key)) {
    // Populate transaction with the sufficient amount of events
    for (int i = 0; i < eventsPerTxn; i++) {
        transaction.writeEvent(key.get(), ByteBuffer.wrap(payload));
    }
}

Now, I assume that ++eventCount >= eventsPerTransaction is going to be true, so the commit will be invoked, right? This means that we can get rid of probeTransaction method. What do you think?

Sure, I think that totally makes sense.
There're 2 reasons why I've put the probe of transaction into a separate method:

1. Clarification. At least for me, that was initially unclear that OpenMessaging framework requests a probe of producer prior to load generation, that's why I've put it into docstrings. However, that could be changed with a single-line comment.
2. Prevention of writing an extra event. We'd write additional event outside of probeness context. However, since we'd write sufficient amount of events with probeness payload into transaction, additional event wouldn't fail it.

if (transaction == null) { ... }
if (includeTimestampInEvent) { ... }