add vendoring with go dep

vendor/github.com/influxdata/influxdb/pkg/estimator/sketch.go

@@ -0,0 +1,18 @@
+package estimator
+
+import "encoding"
+
+// Sketch is the interface representing a sketch for estimating cardinality.
+type Sketch interface {
+	// Add adds a single value to the sketch.
+	Add(v []byte)
+
+	// Count returns a cardinality estimate for the sketch.
+	Count() uint64
+
+	// Merge merges another sketch into this one.
+	Merge(s Sketch) error
+
+	encoding.BinaryMarshaler
+	encoding.BinaryUnmarshaler
+}