add WriterData field to the message struct

[3AceShowHand]

close #1054

This PR adds a new field metadata interface{} into the message struct, which can be used to hold any information, this can be helpful for the application to do post work after confirming the message is delivered.

[3AceShowHand]

@achille-roussel @rhansen2

Please take a look at this PR~

[dominicbarnes]

We're open to merging this, but we'd like to rename this new field to make it's purpose and limitations as clear as possible.

Since Kafka has no built-in notion of "metadata" for messages like this, it only exists in the context of the Writer, as it will not be passed to the broker and thus never consumed downstream. Additionally, "metadata" already has usage within Kafka systems (eg: brokers) so we'd prefer to avoid any confusion there.

Would you mind renaming Metadata to WriterData? I feel like this name will be much less ambiguous, while still accomplishing the same thing for the intended use-cases.

[3AceShowHand]

thanks for your reply, I will update this PR quickly.

[3AceShowHand]

@dominicbarnes please take a look, the field is renamed to WriterData

[3AceShowHand]

@dominicbarnes please take a look at this PR.

It looks the failed one is an unstable case, can you trigger it again?

[3AceShowHand]

@dominicbarnes can you take a look at this issue ? any response is appreciated~

[3AceShowHand]

@achille-roussel @rhansen2 Can you help to take a look at this issue? We hope this can be merged

[dominicbarnes]

I've triggered the failing kafka-011 run again, as I expect it to just be a flaky test as well. I'll review this PR once more with my team today, I expect it will be merged shortly.