Add info about new Repository metrics

This adds more information about the (still in progress)
metrics on Repository for project health.

Author: Eric Sorenson

docs/open-source-health-metrics.md

@@ -41,8 +41,8 @@ You can retrieve information about the [community standards documents](https://d
 
 ### Repository metrics
 
-(coming soon)
+The Repository object in the GraphQL API is the primary location for metrics which relate to project health. These metrics are available in the `Repository` object, and while there are lots of interesting fields available, we've recently coalesced the most useful ones under the `metrics` field. Check the [API docs](https://docs.github.com/en/graphql/reference/objects#repository) for the full list.
 
-### Organization metrics
-
-(coming soon)
+- **LastContributionDate** - The most recent date there was _any of_ the following activity: a commit to a repository’s default branch, opening an issue or discussion, answering a discussion, proposing a pull request, or submitting a pull request review. This is a good single-number metric to find projects that may be unmaintained or in need of archiving.
+- **CommitCount** - A monotonically increasing count of the total number of commits pushed to the default branch of the repository. Tracking the change in this over time will give a sense of the overall activity in the repository.
+- **IssueCommentCount** - Similar to CommitCount, this is a monotonically increasing count of the total number of comments on issues in the repository.