Add metrics spec supplementary guidelines (#1966)
Add metrics spec supplementary guidelines (#1966)

* add metrics spec supplementary guidelines

* fix wording

* tweak wording

* improve readability

* fix typo

* spellcheck

* fix typo

* add more examples

* try to clarify a bit

* improve wording

* cover more topics

* fix typo

* fix typo

* layout

* fix typo

* Update specification/metrics/supplementary-guidelines.md

Co-authored-by: Alan West <52f10591a904eb39daa00ac4135a583e9c63fdfc@users.noreply.github.com>

Co-authored-by: Alan West <52f10591a904eb39daa00ac4135a583e9c63fdfc@users.noreply.github.com>
open-telemetry/opentelemetry-specification,open-telemetry/opentelemetry-specification