Instrument work items to-do icon

Summary

With the release of #390549 (closed), we should instrument this work items to-do feature, especially as we've introduced a new icon and users may struggle to find the feature.

Please follow this instrumentation schema for instrumentation: https://docs.gitlab.com/ee/development/work_items.html#redis-hll-counter-schema

Additional information

Please be aware that the to-do feature is instrumented on both epics and issues, in case this is helpful for reference.

• Epics: click_todo for the Epic show context
• Issues: database counter (Sisense chart)