Describes a rolling range.
[most_recent_span - num_spans + 1, most_recent_span].
For example, say you want the range to include only the latest span,
the appropriate RollingRange would simply be:
RollingRange < num_spans = 1 >
The range is clipped based on available data.
ote that num_spans is required in
proto.RollingRange, while others are optional.