add +active-users (#11)

billing/Earthfile

@@ -0,0 +1,9 @@
+VERSION 0.6
+
+FROM alpine/git:2.36.3
+
+# Estimates monthly active users of a GIT remote repo, based on past commits
+active-users:
+  RUN --secret repoUrl git clone --bare --filter=blob:none $repoUrl repo;
+  RUN cd repo; \
+      git --no-pager log -s --format="%ae" --since=30.days | grep -v @users.noreply.github.com | sort | uniq -c | awk '$1 >= 3 {print $0}' | wc -l

billing/README.md

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+# Billing utilities
+## +active-users
+Estimates monthly active users of a GIT remote repo, based on past commits
+
+Usage: 
+```
+earthly --secret repoUrl=<repoUrl> github.com/earthly/lib/billing+active-users
+```
+
+### Private repositories
+For private repositories use `https` URLs, and pass credentials within the URL:
+```
+earthly --secret repoUrl=https://<user>@<password>/host/repo.git github.com/earthly/lib/billing+active-users 
+```
+> Note:
+> For GitHub private repos you need to use a [personal access token](https://docs.github.com/en/authentication/keeping-your-account-and-data-secure/creating-a-personal-access-token) instead of your password