Reduce allocations during fingerprinting.

Also, add a test to expose
https://github.com/prometheus/client_golang/issues/74 .

benchmark           old ns/op     new ns/op     delta
BenchmarkMetric     7034          6272          -10.83%

benchmark           old allocs     new allocs     delta
BenchmarkMetric     52             32             -38.46%

benchmark           old bytes     new bytes     delta
BenchmarkMetric     1976          1800          -8.91%

model/metric.go

@@ -22,6 +22,8 @@ import (
 	"strings"
 )
 
+var separator = []byte{0}
+
 // A Metric is similar to a LabelSet, but the key difference is that a Metric is
 // a singleton and refers to one and only one stream of samples.
 type Metric map[LabelName]LabelValue
@@ -64,23 +66,34 @@ func (m Metric) String() string {
 
 // Fingerprint returns a Metric's Fingerprint.
 func (m Metric) Fingerprint() Fingerprint {
-	labelLength := len(m)
+	labelNames := make([]string, 0, len(m))
-	labelNames := make([]string, 0, labelLength)
+	maxLength := 0
 
-	for labelName := range m {
+	for labelName, labelValue := range m {
 		labelNames = append(labelNames, string(labelName))
+		if len(labelName) > maxLength {
+			maxLength = len(labelName)
+		}
+		if len(labelValue) > maxLength {
+			maxLength = len(labelValue)
+		}
 	}
 
 	sort.Strings(labelNames)
 
 	summer := fnv.New64a()
+	buf := make([]byte, maxLength)
 
 	for _, labelName := range labelNames {
 		labelValue := m[LabelName(labelName)]
 
-		summer.Write([]byte(labelName))
+		copy(buf, labelName)
-		summer.Write([]byte{0})
+		summer.Write(buf[:len(labelName)])
-		summer.Write([]byte(labelValue))
+
+		summer.Write(separator)
+
+		copy(buf, labelValue)
+		summer.Write(buf[:len(labelValue)])
 	}
 
 	return Fingerprint(binary.LittleEndian.Uint64(summer.Sum(nil)))

model/metric_test.go

@@ -38,6 +38,24 @@ func testMetric(t testing.TB) {
 			},
 			fingerprint: 1470933794305433534,
 		},
+		// The following two demonstrate a bug in fingerprinting. They
+		// should not have the same fingerprint with a sane
+		// fingerprinting function. See
+		// https://github.com/prometheus/client_golang/issues/74 .
+		{
+			input: Metric{
+				"a": "bb",
+				"b": "c",
+			},
+			fingerprint: 3734646176939799877,
+		},
+		{
+			input: Metric{
+				"a":  "b",
+				"bb": "c",
+			},
+			fingerprint: 3734646176939799877,
+		},
 	}
 
 	for i, scenario := range scenarios {