Add metrics for unique and active contributor counts

[d33bs]

Description

This PR adds metrics which count unique contributors in various ways. I use this to help interpret "active contributors" by looking at the number of unique contributors within the last year and 182 days (roughly half a year). In addition, I also added a metadata field which allows the table contents to have a relative datetime stamp which may be useful for understanding the unique contributors per 1 year and per 182 days since the time the table was generated.

Thanks for any feedback!

Closes #159

What is the nature of your change?

• Content additions or updates (adds or updates content)
• Bug fix (fixes an issue).
• Enhancement (adds functionality).
• Breaking change (these changes would cause existing functionality to not work as expected).

Checklist

Please ensure that all boxes are checked before indicating that this pull request is ready for review.

• I have read the CONTRIBUTING.md guidelines.
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• I have made corresponding changes to related documentation (outside of book content).
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[falquaddoomi]

Neat! All the code looks good and makes sense to me, and the metrics you added are useful; kudos.

[d33bs]

Thanks @falquaddoomi ! Going to merge this in.