The allowed types for [VALUE] in a [KEY]:[VALUE] attribute.
This message has oneof_ fields (mutually exclusive fields).
For each oneof, at most one member field can be set at the same time.
Setting any member of the oneof automatically clears all other
members.
A span represents a single operation within a trace. Spans
can be nested to form a trace tree. Often, a trace contains a
root span that describes the end-to-end latency, and one or more
subspans for its sub-operations. A trace can also contain
multiple root spans, or none at all. Spans do not need to be
contiguous—there may be gaps or overlaps between spans in
a trace.