DataDog and StatsD Metrics Support

* Added support for DataDog and StatsD monitoring
* Added documentation

vendor/github.com/go-kit/kit/log/value.go

@@ -0,0 +1,62 @@
+package log
+
+import (
+	"time"
+
+	"github.com/go-stack/stack"
+)
+
+// A Valuer generates a log value. When passed to Context.With in a value
+// element (odd indexes), it represents a dynamic value which is re-evaluated
+// with each log event.
+type Valuer func() interface{}
+
+// bindValues replaces all value elements (odd indexes) containing a Valuer
+// with their generated value.
+func bindValues(keyvals []interface{}) {
+	for i := 1; i < len(keyvals); i += 2 {
+		if v, ok := keyvals[i].(Valuer); ok {
+			keyvals[i] = v()
+		}
+	}
+}
+
+// containsValuer returns true if any of the value elements (odd indexes)
+// contain a Valuer.
+func containsValuer(keyvals []interface{}) bool {
+	for i := 1; i < len(keyvals); i += 2 {
+		if _, ok := keyvals[i].(Valuer); ok {
+			return true
+		}
+	}
+	return false
+}
+
+// Timestamp returns a Valuer that invokes the underlying function when bound,
+// returning a time.Time. Users will probably want to use DefaultTimestamp or
+// DefaultTimestampUTC.
+func Timestamp(t func() time.Time) Valuer {
+	return func() interface{} { return t() }
+}
+
+var (
+	// DefaultTimestamp is a Valuer that returns the current wallclock time,
+	// respecting time zones, when bound.
+	DefaultTimestamp Valuer = func() interface{} { return time.Now().Format(time.RFC3339) }
+
+	// DefaultTimestampUTC is a Valuer that returns the current time in UTC
+	// when bound.
+	DefaultTimestampUTC Valuer = func() interface{} { return time.Now().UTC().Format(time.RFC3339) }
+)
+
+// Caller returns a Valuer that returns a file and line from a specified depth
+// in the callstack. Users will probably want to use DefaultCaller.
+func Caller(depth int) Valuer {
+	return func() interface{} { return stack.Caller(depth) }
+}
+
+var (
+	// DefaultCaller is a Valuer that returns the file and line where the Log
+	// method was invoked. It can only be used with log.With.
+	DefaultCaller = Caller(3)
+)