Based on #17765.
Basically the idea is to reduce the overhead of calling
`ObservableDeferred` in a loop. The two gains are: a) just using a list
of deferreds rather than the machinery of `ObservableDeferred`, and b)
only calling `PreseverLoggingContext` once.
`PreseverLoggingContext` in particular is expensive to call a lot as
each time it needs to call `get_thread_resource_usage` twice, so that it
an update the CPU metrics of the log context.
