* Preserve task result in TaskHandle by keeping join handle around
The solution is not great, but it should hep to debug staging issue
I tried to do it in a least destructive way. TaskHandle used only in
one place so it is ok to use something less generic unless we want
to extend its usage across the codebase. In its current current form
for its single usage place it looks too abstract
Some problems around this code:
1. Task can drop event sender and continue running
2. Task cannot be joined several times (probably not needed,
but still, can be surprising)
3. Had to split task event into two types because ahyhow::Error
does not implement clone. So TaskContinueEvent derives clone
but usual task evend does not. Clone requirement appears
because we clone the current value in next_task_event.
Taking it by reference is complicated.
4. Split between Init and Started is artificial and comes from
watch::channel requirement to have some initial value.
To summarize from 3 and 4. It may be a better idea to use
RWLock or a bounded channel instead
Changes are:
* Correct typo "firts" -> "first"
* Change <empty panic with comment explaining> to <panic with message
taken from the comment>
* Fix weird indentation that rustfmt was failing to handle
* Use existing `anyhow::{anyhow,bail}!` as `{anyhow,bail}!` if it's
already in scope
* Spell `Result<T, anyhow::Error>` as `anyhow::Result<T>`
* In general, closer to matching the rest of the codebase
* Change usages of `hash_map::Entry` to `Entry` when it's already in
scope
* A quick search shows our style on this one varies across the files
it's used in
* Fix extreme metrics bloat in storage sync
From 78 metrics per (timeline, tenant) pair down to (max) 10 metrics per
(timeline, tenant) pair, plus another 117 metrics in a global histogram that
replaces the previous per-timeline histogram.
* Drop image sync operation metric series when dropping TimelineMetrics.
