The watch count metrics are not robust to duplicate cancellations. These
cause the count to be decremented twice, leading eventually to negative
counts. We are seeing this in production. The duplicate cancellations
themselves are not themselves a big problem (except performance), but
they are caused by the new proactive cancellation logic (#11850) which
cancels proactively even immediately before initiating a Close, thus
nearly guaranteeing a Close-cancel race, as discussed in
watchable_store.go. We can avoid this in most cases by not sending a
cancellation when we are going to Close.
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