Add usage statistics of the documentation hosted under /help

The content/docs team has a big issue going on regarding the usage of the built-in docs https://gitlab.com/gitlab-org/gitlab-ce/issues/18739 and wether it should be removed from the Rails app or not.

There are compelling reasons from both sides, so in order to have some concrete data, I'd like to propose we gathered metrics about how often people use GitLab's built-in docs.

Could something like this added in the usage data we already collect? Note that we want to measure /help/* not /help, since the help page also contains information about GitLab's version that people tend to check.

PS. That would be super helpful to be added in CE also, but the usage data was implemented in EE only, so...

cc @regisF