FR: Add precondition support to 'set' methods

[eranl]

The 'update' methods of UpdateBuilder and DocumentReference support a Precondition parameter, while 'set' methods do not, even though they use the same underlying server API, which supports preconditions.

I suggest adding precondition support to 'set' methods as well, as that would be useful for cases where you want to replace a document, unless it has been modified elsewhere since you loaded it.

I have implemented a POC of this feature, and tested that it works. I would be happy to submit a PR.

[dconeybe]

Thanks for the suggestion. It makes sense to me. We have a fairly rigorous process to go through when making changes to the API surface so, despite the simplicity of the requested change, it would take some time to get it through. We'd also want to add this feature to https://github.com/googleapis/nodejs-firestore, for consistency. Finally, we have competing priorities that would bump this feature low on our todo list. But I will bring it up with the team to discuss.

Feel free to open a PR; however, it's not necessary at this point. I assume you're just suggesting adding "precondition" to SetOptions. Is that what you were thinking?

[eranl]

Thanks for the response. I was actually thinking of a separate precondition parameter, similarly to the 'update' methods, but adding it to SetOptions makes sense too.

I would appreciate updates here as you go through the process.

[dconeybe]

After discussing with the team, we remembered that this same feature was requested last year: googleapis/nodejs-firestore#1697. Unfortunately, we still do not have the time to prioritize it now but do think that it is a reasonable request. We will keep you posted if/when we start to work on this.

[eranl]

Thanks for the update! Once you're ready for implementation, let me know if I can help with coding or testing.