Migrate selected user_packages_total_unique_counts_monthly metric to Snowplow

Closes https://gitlab.com/gitlab-org/product-intelligence/-/issues/537

In https://gitlab.com/gitlab-org/product-intelligence/-/issues/493 we selected experimental set of metrics. Now we need to send them using Snowplow. This MR migrates user_packages_total_unique_counts_monthly related metrics