Rename http.* attributes to align with ECS

[trask]

What are you trying to achieve?

Align http.* attributes with corresponding ECS names (see open-telemetry/oteps#222 for motivation).

Rename:

• http.method -> http.request.method
• http.status_code -> http.response.status_code
• http.request_content_length -> http.request.body.bytes
• http.response_content_length -> http.response.body.bytes

Rename http.resend_count to follow above convention, even though ECS has no corresponding attribute yet:

• http.resend_count -> http.request.resend_count