feat: integrate Codecov for coverage reporting

[Pymetheus]

Description

Added Codecov upload steps to CI and PR workflows, created a codecov.yml configuration file, and updated documentation (README, CHECKLIST, INSTRUCTIONS) to reflect Codecov integration. This enables automated cloud-based coverage reporting, PR comments, and visualization of test coverage for improved code quality enforcement.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring
• Performance improvement
• Test addition or update

Related Issue

No issue

Changes Made

• added Codecov upload steps to CI and PR workflows
• created a codecov.yml configuration file
• updated documentation (README, CHECKLIST, INSTRUCTIONS) to reflect Codecov integration

Testing

Describe how you tested your changes:

• Unit tests pass
• Integration tests pass
• Manual testing performed

Screenshots (if applicable)

Add screenshots to help explain your changes.

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation accordingly
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Additional Context

Add any other context about the PR here.

[codecov]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

ℹ️ You can also turn on project coverage checks and project coverage reporting on Pull Request comment

Thanks for integrating Codecov - We've got you covered ☂️