Rename http body size metrics to match attribute names

[trask]

Fixes #121

Changes

Renames http.server.request.size metric to http.server.request.body.size and http.server.response.size metric to http.server.response.body.size.

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• CHANGELOG.md updated for non-trivial changes.
• schema-next.yaml updated with changes to existing conventions.

[reyang]

@trask need to resolve merge conflicts.