Track side bar metrics for a merge request

Task:

Track the following metrics with HLL tracking

Action	Description
time_estimate	Changed the time estimate of the MR
time_spent	Changed the time spent of the MR
lock_added	Locked the MR
lock_removed	Unlocked the MR
label	Changed the labels of the MR
milestone	Changed the milestone of the MR
changed assignee	Changed the assignees of the MR
changed reviewer	Changed the reviewers of the MR