Add prometheus_client service output module, update prometheus client

- Adds a client implementation using the prometheus go_client library
  that exposes metrics.

- Adds a new type of output "ServiceOutput" which follows inline with
  the "ServicePlugin", adding a Stop and Start method for the service

This change also requires the newer prometheus/client_golang code, so
the prometheus plugin needed to be changed.

Added the following to Godep:
    - bitbucket.org/ww/goautoneg (in github.com/common/expfmt/encode.go)
    - prometheus/common/expfmt (in plugins/prometheus.go)
    - github.com/prometheus/common/model (in plugins/prometheus.go)
    - github.com/prometheus/procfs (in github.com/client_golang/prometheus)
    - github.com/beorn7/perks/quantile (in github.com/client_golang/prometheus)

X-Github-Meta: closes #306

plugins/aerospike/aerospike.go

@@ -250,7 +250,8 @@ func get(key []byte, host string) (map[string]string, error) {
 func readAerospikeStats(stats map[string]string, acc plugins.Accumulator, host, namespace string) {
 	for key, value := range stats {
 		tags := map[string]string{
-			"host": host,
+			"host":      host,
+			"namespace": "_service",
 		}
 
 		if namespace != "" {

plugins/prometheus/prometheus.go

@@ -3,13 +3,12 @@ package prometheus
 import (
 	"errors"
 	"fmt"
+	"github.com/influxdb/telegraf/plugins"
+	"github.com/prometheus/common/expfmt"
+	"github.com/prometheus/common/model"
+	"io"
 	"net/http"
 	"sync"
-	"time"
-
-	"github.com/influxdb/telegraf/plugins"
-	"github.com/prometheus/client_golang/extraction"
-	"github.com/prometheus/client_golang/model"
 )
 
 type Prometheus struct {
@@ -60,42 +59,38 @@ func (g *Prometheus) gatherURL(url string, acc plugins.Accumulator) error {
 	if resp.StatusCode != http.StatusOK {
 		return fmt.Errorf("%s returned HTTP status %s", url, resp.Status)
 	}
-	processor, err := extraction.ProcessorForRequestHeader(resp.Header)
-	if err != nil {
-		return fmt.Errorf("error getting extractor for %s: %s", url, err)
+	format := expfmt.ResponseFormat(resp.Header)
+
+	decoder := expfmt.NewDecoder(resp.Body, format)
+
+	options := &expfmt.DecodeOptions{
+		Timestamp: model.Now(),
+	}
+	sampleDecoder := &expfmt.SampleDecoder{
+		Dec:  decoder,
+		Opts: options,
 	}
 
-	ingestor := &Ingester{
-		acc: acc,
-	}
-
-	options := &extraction.ProcessOptions{
-		Timestamp: model.TimestampFromTime(time.Now()),
-	}
-
-	err = processor.ProcessSingle(resp.Body, ingestor, options)
-	if err != nil {
-		return fmt.Errorf("error getting processing samples for %s: %s", url, err)
-	}
-	return nil
-}
-
-type Ingester struct {
-	acc plugins.Accumulator
-}
-
-// Ingest implements an extraction.Ingester.
-func (i *Ingester) Ingest(samples model.Samples) error {
-	for _, sample := range samples {
-		tags := map[string]string{}
-		for key, value := range sample.Metric {
-			if key == model.MetricNameLabel {
-				continue
-			}
-			tags[string(key)] = string(value)
+	for {
+		var samples model.Vector
+		err := sampleDecoder.Decode(&samples)
+		if err == io.EOF {
+			break
+		} else if err != nil {
+			return fmt.Errorf("error getting processing samples for %s: %s", url, err)
+		}
+		for _, sample := range samples {
+			tags := map[string]string{}
+			for key, value := range sample.Metric {
+				if key == model.MetricNameLabel {
+					continue
+				}
+				tags[string(key)] = string(value)
+			}
+			acc.Add(string(sample.Metric[model.MetricNameLabel]), float64(sample.Value), tags)
 		}
-		i.acc.Add(string(sample.Metric[model.MetricNameLabel]), float64(sample.Value), tags)
 	}
+
 	return nil
 }