Add missing example requirements

[pirgeo]

Description

Adds missing requirements.txt for one example (https://github.com/open-telemetry/opentelemetry-python/tree/main/docs/examples/metrics/instruments).

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

pip install -r requirements.txt
python example.py                      # this is already part of the docs

Does This PR Require a Contrib Repo Change?

• [ ] Yes. - Link to PR:
• [x] No.

Checklist:

• [x] Followed the style guidelines of this project
• [ ] ~Changelogs have been updated~
• [ ] ~Unit tests have been added~
• [ ] ~Documentation has been updated~