Include body sizes in outbound HTTP telemetry

[lann]

This would be useful for egress bandwidth accounting. See e.g. https://opentelemetry.io/docs/specs/semconv/registry/attributes/http/#http-request-body-size

The easy version of this would be to just report body sizes based off of content-length headers, which would work for "most" requests. The harder but more complete version would be to use custom Body wrapper(s) to record the actual bytes written, which would additionally work for streaming/chunked-encoding bodies that have no content-length.

[lann]

I just realized that the current span only accounts for the time between when the request is sent and when the response header is received. Any additional time spent receiving the body isn't accounted for. A custom response body wrapper would also allow us to rectify that.