googMetrics are non-standard drafts introduced to getStats() by Google Developers to easily test new metrics. Since these metrics are non-standard, experimental metrics, they are not meant to be included in the standard output of getStats().
getStats() sets strict standards and a high bar for metrics. Metrics should be well-defined and documented in a reasonable way, so implementations may actualize them independently without looking at each others code.
As is typical with experimental metrics, they are not well-defined, especially when in the process of experimentation. When these metrics are exposed in a standard API, developers tend to start using them and it can be difficult to remove them. Several apps or products may come to rely on an experimental metric that was never meant to be included in the standard output.