metrics: flesh out some of the metric types

src/runtime/metrics/metrics.go

@@ -2,15 +2,26 @@
 
 package metrics
 
-type Description struct{}
+type Description struct {
+	Name        string
+	Description string
+	Kind        ValueKind
+	Cumulative  bool
+}
 
 func All() []Description {
 	return nil
 }
 
-type Float64Histogram struct{}
+type Float64Histogram struct {
+	Counts  []uint64
+	Buckets []float64
+}
 
-type Sample struct{}
+type Sample struct {
+	Name  string
+	Value Value
+}
 
 func Read(m []Sample) {}
 
@@ -23,10 +34,17 @@ func (v Value) Float64Histogram() *Float64Histogram {
 	return nil
 }
 func (v Value) Kind() ValueKind {
-	return ValueKind{}
+	return KindBad
 }
 func (v Value) Uint64() uint64 {
 	return 0
 }
 
-type ValueKind struct{}
+type ValueKind int
+
+const (
+	KindBad ValueKind = iota
+	KindUint64
+	KindFloat64
+	KindFloat64Histogram
+)