Cosmos: Restore PATCH API

[analogrelay]

In #3764 , we temporarily removed the PATCH API while we revisit idempotency in the context of enabling automatic retries for all write operations. Since all other operations are generally retry-safe as long as the user accounts for conflict errors, the concern with PATCH was that (the increment in particular) may silently double-count in the context of an automatic retry for a write operation.

Our decision after reviewing things is to accept this risk and document it clearly. We've heard enough feedback from customers that PATCH is still useful even in the context of an increased RU cost and potential idempotency issues. Actually, we're still deferring this until post-GA. I mis-remembered some past context :). Just making that clear.

We'll be working to investigate a model that allows PATCH to be idempotent.

[FabianMeiswinkel]

@analogrelay
I DO NOT think we have a quorum there yet. Customers who do not want to worry about the etag dance I clearly see - but enabling retries AND ignoring idempotency will result in confusion and misunderstandings - same as not retrying at all. I think it is worth to spend some time to come up with a model that allows Patch to be auto-retried AND idempotent - and if that is not possible at least force an intentional decision whether to prioritize idempotency or auto-retry - only way to avoid confusion is to force that decision.

[analogrelay]

Yep, I remember now, we landed on deferring past GA for now. Thanks for the reminder! I've updated the issue