Hello.
While working on #1854 I ran into a few typing issues that got in the way:

1. Most types like Counter, Gauge, Histogram, and Summary have both an interface and a concrete private implementation (counter, gauge, etc.) that implements it. But for the vector types (CounterVec, GaugeVec) there’s no interface — they’re implemented directly. I introduced interfaces for these vector types and moved the implementations into private types (counterVec, gaugeVec).

2. The ObserverVec interface for HistogramVec and SummaryVec implements all its methods and also satisfies Collector. However, for Histogram and Summary, the Observer interface only defines Observe(float64). This prevents creating a generic interface for Histogram and Summary in places that require a Collector implementation. I added the missing methods to Observer and introduced a new ObserverMetod interface.

3. Added the missing methods to ObserverVec.

4. All *Vec types use MetricVec under the hood. I also turned MetricVec into an interface.

5. I tried to maintain maximum backward compatibility to minimize the likelihood of changes in existing projects.
   In most cases, everything will work without changes or with minimal changes (for example, replacing type from pointer to the interface -- *CounerVec => CounerVec)