When a stream advances in the database but stops being replicated to a
process, that process's view of the stream freezes. Requests that wait
for it to catch up to a token issued by another worker then time out and
return empty responses indefinitely (see #20080), and nothing exported
said so.
Report `get_current_token` from every ID generator, on every process.
The value is comparable between processes, so a stream that has stopped
reaching one of them shows up as divergence with no client traffic
needed. It is also the position that `wait_for_stream_token` waits on,
so its divergence is the failure itself rather than a proxy for it.
Being a watermark over gapless runs of persisted IDs, it also catches a
single writer of a sharded stream going quiet, which a maximum across
writers would hide behind the writers still being replicated.
Erik Johnston
f3ae564877