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]

Just caught up on the effect of open-telemetry/opentelemetry-specification#3355 which. The ECS names for the fields I'd like are:

• http.request.bytes
• http.response.bytes

By analogy with the renamings introduced in that PR, the new attributes should be

• http.request.size
• http.response.size

[trask]

just adding the same heads up here from #84 (comment) (for anyone else viewing the issue):

the HTTP semantic convention WG isn't considering any additions until the HTTP semantic conventions have at least been marked Frozen, so it may be a few weeks until we get to reviewing this proposal