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

outputs/prometheus_client/prometheus_client.go

@@ -0,0 +1,109 @@
+package prometheus_client
+
+import (
+	"fmt"
+	"github.com/influxdb/influxdb/client/v2"
+	"github.com/influxdb/telegraf/outputs"
+	"github.com/prometheus/client_golang/prometheus"
+	"net/http"
+)
+
+type PrometheusClient struct {
+	Listen  string
+	server  *http.Server
+	metrics map[string]*prometheus.UntypedVec
+}
+
+var sampleConfig = `
+  # Address to listen on
+  # listen = ":9126"
+`
+
+func (p *PrometheusClient) Start() error {
+	if p.Listen == "" {
+		p.Listen = ":9126"
+	}
+	http.Handle("/metrics", prometheus.Handler())
+	server := &http.Server{
+		Addr:    p.Listen,
+		Handler: prometheus.Handler(),
+	}
+	p.server = server
+	p.metrics = make(map[string]*prometheus.UntypedVec)
+	go p.server.ListenAndServe()
+	return nil
+}
+
+func (p *PrometheusClient) Stop() {
+	// TODO: Use a listener for http.Server that counts active connections
+	//       that can be stopped and closed gracefully
+}
+
+func (p *PrometheusClient) Connect() error {
+	// This service output does not need to make any further connections
+	return nil
+}
+
+func (p *PrometheusClient) Close() error {
+	// This service output does not need to close any of its connections
+	return nil
+}
+
+func (p *PrometheusClient) SampleConfig() string {
+	return sampleConfig
+}
+
+func (p *PrometheusClient) Description() string {
+	return "Configuration for the Prometheus client to spawn"
+}
+
+func (p *PrometheusClient) Write(points []*client.Point) error {
+	if len(points) == 0 {
+		return nil
+	}
+
+	for _, point := range points {
+		var labels []string
+		name := point.Name()
+		key := name
+
+		for k, _ := range point.Tags() {
+			if len(k) > 0 {
+				labels = append(labels, k)
+			}
+		}
+
+		if _, ok := p.metrics[key]; !ok {
+			p.metrics[key] = prometheus.NewUntypedVec(
+				prometheus.UntypedOpts{
+					Name: key,
+					Help: fmt.Sprintf("Telegraf collected point '%s'", name),
+				},
+				labels,
+			)
+			prometheus.MustRegister(p.metrics[key])
+		}
+
+		l := prometheus.Labels{}
+		for tk, tv := range point.Tags() {
+			l[tk] = tv
+		}
+
+		for _, val := range point.Fields() {
+			switch val.(type) {
+			case int64:
+				ival := val.(int64)
+				p.metrics[key].With(l).Set(float64(ival))
+			case float64:
+				p.metrics[key].With(l).Set(val.(float64))
+			}
+		}
+	}
+	return nil
+}
+
+func init() {
+	outputs.Add("prometheus_client", func() outputs.Output {
+		return &PrometheusClient{}
+	})
+}