SetCustomVariables for Piwik adapter

Does the `setCustomVarible` will be implemented for matomo adapter? Or the fast way should be define my own adapter with this function.

Ref: https://github.com/matomo-org/matomo/blob/295b62fd06333c8a5ef42603d096978d73e3b282/js/piwik.js#L5507

Start by finding the existing Matomo/Piwik adapter in ember-metrics and comparing its supported calls with the linked piwik.js setCustomVariable reference. Also look for other adapters that expose custom-variable-like methods. Done would mean the Matomo adapter has documented support for this call and matching tests, but the issue has had no maintainer response for years.

索引モデルが issue の本文から書いたものです。