Add getValue() method to Counter type metric

[im-konge]

Description

The Counter metric type is the only one missing the getValue() method, which is used for obtaining the value parameter from the Counter object.
This PR adds it to the code.

Type of Change

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

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• 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/integration tests pass locally with my changes