Fix the established session metric

This metric shouldn't include delays that may happen due to the user's behaviour (establishing an SSH connection may require user's input like passphrase). It should include only app-related delays.

Related issue: Strategy to deploy the newest version of Gitlab... (#670 - closed)

Apdex relies on the following metric: gitlab_shell_sshd_session_established_duration_seconds_bucket

And after 59224334 it changed slightly indeed: