Fix bug with Danger bot for Analytics Instrumentation

Fixes bug discovered in !153401 (comment 1918282771).

The caused by helper.milestone being hash object instead of string`.

To verify the fix see !154059 (closed)

Description of the rule being fixed:

This code change updates the analytics instrumentation for a Dangerfile, which is a tool that automates tasks in a GitLab merge request. The update ensures that when a metric is removed from a configuration file, the correct milestone is associated with the removal. Additionally, it provides a more user-friendly error message when a milestone is not set.