Add http metrics yaml

[trask]

Fixes #2972 by adding text to say whether each instrument is required or optional (ideally should be added to yaml?)
Fixes #2822
Fixes #2974 by including same definition of net.host.name from tracing
Fixes #2976 by making http.server.request.size and friends optional

Changes

Adds http metrics yaml file based on @jamesmoessis build tool changes in open-telemetry/build-tools#79

For non-trivial changes, follow the change proposal process and link to the related issue(s) and/or OTEP(s), update the CHANGELOG.md, and also be sure to update spec-compliance-matrix.md if necessary.

Related issues #2666, #2001

NOTE: Markdown rendering will be improved in a future PR, after open-telemetry/build-tools#129

[reyang]

LGTM considering the semantic convention is still Experimental. There are couple things which we might change (e.g. do we use ms as unit or s), not real blocker here.

[jsuereth]

+1 on adding the notion of optional instruments to the YAML. Is there a ticket open for this?

[trask]

+1 on adding the notion of optional instruments to the YAML. Is there a ticket open for this?

open-telemetry/build-tools#132