Clarify metric attributes should be namespaced

[joaopgrassi]

Fixes #394

Changes

Adds a section on the general metric guidelines about attributes. Specifically, calling out the TC decision about metric attributes being required to be namespaced.

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• schema-next.yaml updated with changes to existing conventions.

[arminru]

LGTM!

I think this would still be worth an entry in the changelog to make others aware.

[joaopgrassi]

@arminru @ChrsMark @trask I reset your approvals since I changed the wording according to our discussion in our last sig meeting. Can you please take another look? Thank you!

[lmolkova]

Left a comment on must vs should, but looks great otherwise