Missing http response size metrics

Due to a missing MustRegister(), the gitlab_workhorse_http_response_duration_seconds_... histogram metric is missing.

mentioned in commit 0e630643

mentioned in merge request !660 (merged)

mentioned in commit cb708c3a

mentioned in commit 35403c6c

mentioned in commit a2bc2f4b

mentioned in commit be3c45d1

mentioned in commit 2caaafca

mentioned in commit d7e07e91

mentioned in commit fa659fc3

closed with merge request !660 (merged)

mentioned in commit 5b34783e

assigned to @bjk-gitlab