Comment cache field of inserter

sdk/metric/pipeline.go

@@ -179,6 +179,18 @@ func (p *pipeline) produce(ctx context.Context) (metricdata.ResourceMetrics, err
 // inserter facilitates inserting of new instruments from a single scope into a
 // pipeline.
 type inserter[N int64 | float64] struct {
+	// cache holds the aggregators this inserter has created and the views the
+	// parent meter resolved.
+	//
+	// The aggregators part of the cache ensures that if this inserter has
+	// already created an aggregator and registered it with the underlying
+	// pipeline (and likewise its underlying reader), that aggregator is
+	// returned if a user asks the meter for an equivalent stream (directly, or
+	// via a view transform).
+	//
+	// The views part ensures that any instrument conflicts (i.e. same name but
+	// different description, unit, ...) are logged. This is done from the
+	// scope of the Meter by virtue of vc coming from the meter.
 	cache    instrumentCache[N]
 	pipeline *pipeline
 }