Respect original visibility for instrumented methods

What does this MR do?

Related issue: #238949 (closed)

After fixing, private methods remain private

list_service = Boards::ListService.new Project.first, User.first
Loading development environment (Rails 6.0.3.1)
[1] pry(main)> list_service = Boards::ListService.new Project.first, User.first
  Project Load (3.7ms)  SELECT "projects".* FROM "projects" ORDER BY "projects"."id" ASC LIMIT $1  [["LIMIT", 1]]
  User Load (13.5ms)  SELECT "users".* FROM "users" ORDER BY "users"."id" ASC LIMIT $1  [["LIMIT", 1]]
  Route Load (3.0ms)  SELECT "routes".* FROM "routes" WHERE "routes"."source_id" = $1 AND "routes"."source_type" = $2 LIMIT $3  [["source_id", 1], ["source_type", "Project"], ["LIMIT", 1]]
=> #<Boards::ListService:0x00007fa7c8605c80 @current_user=#<User id:1 @root>, @params={}, @parent=#<Project id:1 gitlab-org/gitlab-test>>>
[2] pry(main)> list_service.boards
NoMethodError: private method `boards' called for #<Boards::ListService:0x00007fa7c8605c80>
from (pry):2:in `<main>'

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.