We can observe the delay in the metric histogram. Because we run in a synctest
bubble, the delay is 100% predictable.
Unfortunately we cannot use the reactor mechanism of the fake client: that
delays while holding the fake's mutex. When some other goroutine (in this case,
the event recorder) calls the client, it gets blocked without being considered
durably blocked by synctest, so time does not advance and the test gets stuck.
fb94a99d2f