Add benchmark to unmarshal

```
goos: linux
goarch: amd64
pkg: github.com/prometheus/client_golang/api/prometheus/v1
BenchmarkSerialization/10/10/unmarshal/encoding/json-4         	  200000	     10272 ns/op	     232 B/op	       5 allocs/op
BenchmarkSerialization/10/10/unmarshal/jsoniter-4              	10000000	       154 ns/op	      16 B/op	       1 allocs/op
BenchmarkSerialization/10/100/unmarshal/encoding/json-4        	   20000	     87834 ns/op	     232 B/op	       5 allocs/op
BenchmarkSerialization/10/100/unmarshal/jsoniter-4             	10000000	       178 ns/op	      16 B/op	       1 allocs/op
BenchmarkSerialization/10/1000/unmarshal/encoding/json-4       	    2000	    966855 ns/op	     232 B/op	       5 allocs/op
BenchmarkSerialization/10/1000/unmarshal/jsoniter-4            	 5000000	       211 ns/op	      16 B/op	       1 allocs/op
BenchmarkSerialization/100/10/unmarshal/encoding/json-4        	   10000	    124487 ns/op	     232 B/op	       5 allocs/op
BenchmarkSerialization/100/10/unmarshal/jsoniter-4             	10000000	       217 ns/op	      16 B/op	       1 allocs/op
BenchmarkSerialization/100/100/unmarshal/encoding/json-4       	    1000	   1335330 ns/op	     232 B/op	       5 allocs/op
BenchmarkSerialization/100/100/unmarshal/jsoniter-4            	10000000	       184 ns/op	      16 B/op	       1 allocs/op
BenchmarkSerialization/100/1000/unmarshal/encoding/json-4      	     100	  11345940 ns/op	     232 B/op	       5 allocs/op
BenchmarkSerialization/100/1000/unmarshal/jsoniter-4           	 5000000	       219 ns/op	      16 B/op	       1 allocs/op
BenchmarkSerialization/1000/10/unmarshal/encoding/json-4       	    1000	   1093244 ns/op	     232 B/op	       5 allocs/op
BenchmarkSerialization/1000/10/unmarshal/jsoniter-4            	10000000	       170 ns/op	      16 B/op	       1 allocs/op
BenchmarkSerialization/1000/100/unmarshal/encoding/json-4      	     200	   9597895 ns/op	     232 B/op	       5 allocs/op
BenchmarkSerialization/1000/100/unmarshal/jsoniter-4           	10000000	       180 ns/op	      16 B/op	       1 allocs/op
BenchmarkSerialization/1000/1000/unmarshal/encoding/json-4     	      10	 104403258 ns/op	     232 B/op	       5 allocs/op
BenchmarkSerialization/1000/1000/unmarshal/jsoniter-4          	10000000	       176 ns/op	      16 B/op	       1 allocs/op
PASS
ok  	github.com/prometheus/client_golang/api/prometheus/v1	35.976s
```

Signed-off-by: Thomas Jackson <jacksontj.89@gmail.com>

api/prometheus/v1/api_bench_test.go

@@ -0,0 +1,82 @@
+// Copyright 2017 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package v1
+
+import (
+	"encoding/json"
+	"fmt"
+	"strconv"
+	"testing"
+	"time"
+
+	jsoniter "github.com/json-iterator/go"
+	"github.com/prometheus/common/model"
+)
+
+func generateData(timeseries, datapoints int) model.Matrix {
+	// Create the top-level matrix
+	m := make(model.Matrix, 0)
+
+	for i := 0; i < timeseries; i++ {
+		lset := map[model.LabelName]model.LabelValue{
+			model.MetricNameLabel: model.LabelValue("timeseries_" + strconv.Itoa(i)),
+		}
+		now := model.Now()
+		values := make([]model.SamplePair, datapoints)
+
+		for x := datapoints; x > 0; x-- {
+			values[x-1] = model.SamplePair{
+				Timestamp: now.Add(time.Second * -15 * time.Duration(x)), // Set the time back assuming a 15s interval
+				Value:     model.SampleValue(float64(x)),
+			}
+		}
+
+		ss := &model.SampleStream{
+			Metric: model.Metric(lset),
+			Values: values,
+		}
+
+		m = append(m, ss)
+	}
+	return m
+}
+
+func BenchmarkSerialization(b *testing.B) {
+	for _, timeseriesCount := range []int{10, 100, 1000} {
+		for _, datapointCount := range []int{10, 100, 1000} {
+			b.Run(fmt.Sprintf("%d/%d", timeseriesCount, datapointCount), func(b *testing.B) {
+				data := generateData(timeseriesCount, datapointCount)
+
+				dataBytes, err := json.Marshal(data)
+				if err != nil {
+					b.Fatalf("Error marshaling: %v", err)
+				}
+
+				b.Run("unmarshal", func(b *testing.B) {
+					var m model.Matrix
+					b.Run("encoding/json", func(b *testing.B) {
+						for i := 0; i < b.N; i++ {
+							json.Unmarshal(dataBytes, m)
+						}
+					})
+
+					b.Run("jsoniter", func(b *testing.B) {
+						for i := 0; i < b.N; i++ {
+							jsoniter.Unmarshal(dataBytes, m)
+						}
+					})
+				})
+			})
+		}
+	}
+}