Use the `ServeHTTP` plugin hook to create an endpoint in the plugin that will do the following:
Check if the user has a canary cookie set. If they do not, pick a random number between 1 and 100. If the number is above the percentage set in the configuration, set a canary cookie with the value "never". If the number is at the percentage or below, set a canary cookie with the value "always".
The cookie should expire after 24 hours.
See https://github.com/jwilander/mattermost-plugin-docup/blob/master/server/plugin.go#L52 for an example on how to use `ServeHTTP`.
You can test the endpoint by curl'ing it at `<your-mm-url>/plugin/<plugin-id>`