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~

[pirgeo]

I tried to align with the other examples, which all have requirements.txt files. I don't have a preference for either, so I am happy to change it to simply adding it to the readme.

[xrmx]

I tried to align with the other examples, which all have requirements.txt files. I don't have a preference for either, so I am happy to change it to simply adding it to the readme.

Given the status of these requirements I think I have a stronger opinion now :sweat_smile: But let's see what maintainers think.

[lzchen]

I tried to align with the other examples, which all have requirements.txt files. I don't have a preference for either, so I am happy to change it to simply adding it to the readme.

Given the status of these requirements I think I have a stronger opinion now 😅 But let's see what maintainers think.

+1 for this, less maintenence the better :)

[ocelotl]

I suggest we keep the requirements.txt file, just remove the pinned versions.

[pirgeo]

I updated it to use >= instead of ==. Please let me know if you think this works, otherwise I'll just drop the versions completely!

[lzchen]

@ocelotl

I suggest we keep the requirements.txt file, just remove the pinned versions.

We don't requirements.txt file for the examples. Do we really want to start adding them for each example? This seems like more maintenance overhead.