A metric is a measure that can be aggregated over time. You can derive a metric from logs (count) but that doesn't make them the same thing. Spans have start and end times and parent pointer. Logs are arbitrary data, but can be correlated, can have application/org defined structure that's beyond the scope of the observability stack.

You could draw a class diagram with "Wide" - or whatever - as the base class, metric, span, log subclasses. Metric class requires an additional metric field, span requires start, end, parent, log is just open.