Using the semi-secret `within` matcher in RSpec

Sometimes, it can be tricky to write asserts for values that are nondeterministic, but flicker by a negligible amount, especially when working with time.

While I agree that the first line of defence should be to seek to freeze the time, sometimes it's not an option. Enter the "fuzzy" within matcher.

# bad
is_expected.to(
  be < execution_start.since(5.seconds)
  .and(be >= execution_start)
)

# good
is_expected.to be_within(5.seconds).of(execution_start)

Docs on this matcher are rather limited and fail to mention that it can also be used in collections and arguments by omitting the be_ part.

is_expected.to(
  change { record.field }.to(within(5.seconds).of(some_time))
)

expect(SomeClass).to(
  have_received(:call)
  .with(stamp: within(5.seconds).of(some_time))
  .once
)