Fix units of process.open_file_descriptor.count and process.context_switches

[ChrsMark]

Fixes #1662

Changes

This PR fixes the units of process.open_file_descriptor.count and process.context_switches metrics.
I was not sure if it should be {open_file_descriptor} or just {file_descriptor} but went with what was proposed in #1662.

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.

[braydonk]

I believe {open_file_descriptor} is correct as a unit to match the name. This does make me wonder if the word open even needs to be in the name though; file_descriptor itself implies that it's open. Let's merge this though, and I'll make a separate issue for that now.

[ChrsMark]

@open-telemetry/specs-semconv-maintainers shall we merge this?