trace/http: Add semantic convention for total request and response length

[kamalmarhubi]

The current http.request_content_length and http.response_content_length are specified to be the length of the body excluding headers. I'd like to add an attribute for the total message size.

For names, I think http.request_length and http.response_length make the most sense.

Rationale:

• the seemingly related metrics http.server.request.size and http.server.response.size are in fact measure of total bytes sent
• some systems only provide the total HTTP message length, eg Google Cloud Load Balancer logs and AWS ALB logs; these are logs, but we process those logs into spans

[kamalmarhubi]

I didn't see #3489. Reopened as open-telemetry/semantic-conventions#38.