DataDog and StatsD Metrics Support

* Added support for DataDog and StatsD monitoring
* Added documentation

vendor/github.com/go-kit/kit/metrics/internal/ratemap/ratemap.go

@@ -0,0 +1,40 @@
+// Package ratemap implements a goroutine-safe map of string to float64. It can
+// be embedded in implementations whose metrics support fixed sample rates, so
+// that an additional parameter doesn't have to be tracked through the e.g.
+// lv.Space object.
+package ratemap
+
+import "sync"
+
+// RateMap is a simple goroutine-safe map of string to float64.
+type RateMap struct {
+	mtx sync.RWMutex
+	m   map[string]float64
+}
+
+// New returns a new RateMap.
+func New() *RateMap {
+	return &RateMap{
+		m: map[string]float64{},
+	}
+}
+
+// Set writes the given name/rate pair to the map.
+// Set is safe for concurrent access by multiple goroutines.
+func (m *RateMap) Set(name string, rate float64) {
+	m.mtx.Lock()
+	defer m.mtx.Unlock()
+	m.m[name] = rate
+}
+
+// Get retrieves the rate for the given name, or 1.0 if none is set.
+// Get is safe for concurrent access by multiple goroutines.
+func (m *RateMap) Get(name string) float64 {
+	m.mtx.RLock()
+	defer m.mtx.RUnlock()
+	f, ok := m.m[name]
+	if !ok {
+		f = 1.0
+	}
+	return f
+}