Update merge request metrics description

config/metrics/counts_28d/20210216175055_merge_requests.yml

 ---
 data_category: optional
 key_path: usage_activity_by_stage_monthly.create.merge_requests
-description: Count of the number of users creating merge requests
+description: Count distinct authors of merge requests
 product_section: dev
 product_stage: create
 product_group: group::code review

config/metrics/counts_28d/20210216175101_merge_requests_users.yml

 ---
 data_category: optional
 key_path: usage_activity_by_stage_monthly.create.merge_requests_users
-description: Monthly count of the number of merge request users
+description: Distinct count of users performing merge request actions like closed, merged, created, commented
 product_section: dev
 product_stage: create
 product_group: group::code review