Remove unused methods from comment_client
Remove unused *args and **kwargs from perform_request
Use requests library utility method for getting json results
Add more tags to comment_client request metrics

This captures real-time metrics for all of the comment client actions,
segregated by course_id, as well as other small-cardinality fields.

The goal is to be able to detect changes in forum usage, with the goal
of alerting on potential error conditions.