Http semantic convention defines 4 attributes which are very different from all others. Namely http.request_content_length, http.request_content_length_uncompressed, http.response_content_length and http.response_content_length_uncompressed. I think this information should be captured as metrics, not as span attributes. I cannot imagine a use-case where anybody would want to search/filter by this attribute or use it in any other "dimension"-like way. This is clearly a metric.