[DOCS-12762, DOCS-12761] Update IBM Db2 metric description and Python instructions

[evazorro]

What does this PR do?

Updates the description of one of this integration's metrics with suggested wording from the ticket reporter, and corrects a mention of version pinning for Python in the README.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[github-actions]

⚠️ Recommendation: Add qa/skip-qa label

This PR does not modify any files shipped with the agent.

To help streamline the release process, please consider adding the qa/skip-qa label if these changes do not require QA testing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.02%. Comparing base (a5342b2) to head (acd1803).
⚠️ Report is 14 commits behind head on master.

Additional details and impacted files

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