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_test.go

@@ -0,0 +1,90 @@
+package prometheus_client
+
+import (
+	"github.com/influxdb/influxdb/client/v2"
+	"github.com/influxdb/telegraf/plugins/prometheus"
+	"github.com/influxdb/telegraf/testutil"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"testing"
+)
+
+var pTesting *PrometheusClient = &PrometheusClient{}
+
+func TestPrometheusStart(t *testing.T) {
+	require.NoError(t, pTesting.Start())
+}
+
+func TestPrometheusWritePointEmptyTag(t *testing.T) {
+	if testing.Short() {
+		t.Skip("Skipping integration test in short mode")
+	}
+	p := &prometheus.Prometheus{
+		Urls: []string{"http://" + testutil.GetLocalHost() + ":9126/metrics"},
+	}
+	tags := make(map[string]string)
+	var points = []*client.Point{
+		client.NewPoint(
+			"test_point_1",
+			tags,
+			map[string]interface{}{"value": 0.0}),
+		client.NewPoint(
+			"test_point_2",
+			tags,
+			map[string]interface{}{"value": 1.0}),
+	}
+	require.NoError(t, pTesting.Write(points))
+
+	expected := []struct {
+		name  string
+		value float64
+		tags  map[string]string
+	}{
+		{"test_point_1", 0.0, tags},
+		{"test_point_2", 1.0, tags},
+	}
+
+	var acc testutil.Accumulator
+
+	require.NoError(t, p.Gather(&acc))
+	for _, e := range expected {
+		assert.NoError(t, acc.ValidateValue(e.name, e.value))
+	}
+}
+
+func TestPrometheusWritePointTag(t *testing.T) {
+	if testing.Short() {
+		t.Skip("Skipping integration test in short mode")
+	}
+	p := &prometheus.Prometheus{
+		Urls: []string{"http://" + testutil.GetLocalHost() + ":9126/metrics"},
+	}
+	tags := make(map[string]string)
+	tags["testtag"] = "testvalue"
+	var points = []*client.Point{
+		client.NewPoint(
+			"test_point_3",
+			tags,
+			map[string]interface{}{"value": 0.0}),
+		client.NewPoint(
+			"test_point_4",
+			tags,
+			map[string]interface{}{"value": 1.0}),
+	}
+	require.NoError(t, pTesting.Write(points))
+
+	expected := []struct {
+		name  string
+		value float64
+	}{
+		{"test_point_3", 0.0},
+		{"test_point_4", 1.0},
+	}
+
+	var acc testutil.Accumulator
+
+	require.NoError(t, p.Gather(&acc))
+	for _, e := range expected {
+		assert.True(t, acc.CheckTaggedValue(e.name, e.value, tags))
+	}
+}