A trace describes how long it takes for an application to
perform an operation. It consists of a set of spans, each of
which represent a single timed event within the operation.
A span represents a single timed event within a trace. Spans
can be nested and form a trace tree. Often, a trace contains a
root span that describes the end-to-end latency of an operation
and, optionally, one or more subspans for its suboperations.
Spans do not need to be contiguous. There may be gaps between
spans in a trace.
