gitlab-time-tracking-button автор Aleksandar Dimitrov

Adds a time tracking button to easily track your time in Gitlab.

You need to provide permissions and an API key first. Obtain an API key from your Gitlab instance (Settings → Access Tokens → make sure you check 'api'.)
Then use the Addon Preferences to add your instance and the API key.

You're done :-) When you load an issue in your instance, you should see the time tracker in the right sidebar. You can start/stop, save and discard the tracked time from there. Time tracking info is stored in your profile and persists over tabs & browser sessions, so you shouldn't lose any time.