Implement Prometheus factory and config V2

Github issue: open-telemetry#38

Testing done:

- make && make otelsvc
- Run otelsvc with the following config and make sure Prometheus can scrape :

receivers:
  opencensus:
    port: 55678

  prometheus:
    config:
      scrape_configs:
        - job_name: 'demo'
          scrape_interval: 5s

zpages:
  port: 55679

exporters:
  prometheus:
    endpoint: "127.0.0.1:8889"

pipelines:
  metrics:
    receivers: [prometheus]
    exporters: [prometheus]

cmd/occollector/app/builder/receivers_builder.go

@@ -203,7 +203,7 @@ func (rb *ReceiversBuilder) attachReceiverToPipelines(
 				dataType.GetString(),
 				dataType.GetString())
 		}
-		return fmt.Errorf("cannot create receiver %s", config.Name())
+		return fmt.Errorf("cannot create receiver %s: %s", config.Name(), err.Error())
 	}
 
 	rb.logger.Info("Receiver is enabled.",

exporter/prometheusexporter/config.go

@@ -0,0 +1,35 @@
+// Copyright 2019, OpenTelemetry 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 prometheusexporter
+
+import (
+	prometheus_golang "github.com/prometheus/client_golang/prometheus"
+
+	"github.com/open-telemetry/opentelemetry-service/models"
+)
+
+// ConfigV2 defines configuration for Prometheus exporter.
+type ConfigV2 struct {
+	models.ExporterSettings `mapstructure:",squash"` // squash ensures fields are correctly decoded in embedded struct.
+
+	// The address on which the Prometheus scrape handler will be run on.
+	Endpoint string `mapstructure:"endpoint"`
+
+	// Namespace if set, exports metrics under the provided value.
+	Namespace string `mapstructure:"namespace"`
+
+	// ConstLabels are values that are applied for every exported metric.
+	ConstLabels prometheus_golang.Labels `mapstructure:"const_labels"`
+}

exporter/prometheusexporter/config_test.go

@@ -0,0 +1,57 @@
+// Copyright 2019, OpenTelemetry 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 prometheusexporter
+
+import (
+	"path"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/open-telemetry/opentelemetry-service/configv2"
+	"github.com/open-telemetry/opentelemetry-service/factories"
+	"github.com/open-telemetry/opentelemetry-service/models"
+)
+
+var _ = configv2.RegisterTestFactories()
+
+func TestLoadConfig(t *testing.T) {
+	factory := factories.GetExporterFactory(typeStr)
+
+	config, err := configv2.LoadConfigFile(t, path.Join(".", "testdata", "config.yaml"))
+
+	require.NoError(t, err)
+	require.NotNil(t, config)
+
+	e0 := config.Exporters["prometheus"]
+	assert.Equal(t, e0, factory.CreateDefaultConfig())
+
+	e1 := config.Exporters["prometheus/2"]
+	assert.Equal(t, e1,
+		&ConfigV2{
+			ExporterSettings: models.ExporterSettings{
+				NameVal: "prometheus/2",
+				TypeVal: "prometheus",
+				Enabled: true,
+			},
+			Endpoint:  "1.2.3.4:1234",
+			Namespace: "test-space",
+			ConstLabels: map[string]string{
+				"label1":        "value1",
+				"another label": "spaced value",
+			},
+		})
+}

exporter/prometheusexporter/factory.go

@@ -0,0 +1,98 @@
+// Copyright 2019, OpenTelemetry 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 prometheusexporter
+
+import (
+	"net"
+	"net/http"
+	"strings"
+
+	"go.uber.org/zap"
+
+	"github.com/open-telemetry/opentelemetry-service/consumer"
+	"github.com/open-telemetry/opentelemetry-service/factories"
+	"github.com/open-telemetry/opentelemetry-service/models"
+	"github.com/orijtech/prometheus-go-metrics-exporter"
+)
+
+var _ = factories.RegisterExporterFactory(&exporterFactory{})
+
+const (
+	// The value of "type" key in configuration.
+	typeStr = "prometheus"
+)
+
+// exporterFactory is the factory for Prometheus exporter.
+type exporterFactory struct {
+}
+
+// Type gets the type of the Exporter config created by this factory.
+func (f *exporterFactory) Type() string {
+	return typeStr
+}
+
+// CreateDefaultConfig creates the default configuration for exporter.
+func (f *exporterFactory) CreateDefaultConfig() models.Exporter {
+	return &ConfigV2{
+		ExporterSettings: models.ExporterSettings{
+			TypeVal: typeStr,
+			NameVal: typeStr,
+		},
+		ConstLabels: map[string]string{},
+	}
+}
+
+// CreateTraceExporter creates a trace exporter based on this config.
+func (f *exporterFactory) CreateTraceExporter(logger *zap.Logger, config models.Exporter) (consumer.TraceConsumer, factories.StopFunc, error) {
+	return nil, nil, models.ErrDataTypeIsNotSupported
+}
+
+// CreateMetricsExporter creates a metrics exporter based on this config.
+func (f *exporterFactory) CreateMetricsExporter(logger *zap.Logger, cfg models.Exporter) (consumer.MetricsConsumer, factories.StopFunc, error) {
+	pcfg := cfg.(*ConfigV2)
+
+	addr := strings.TrimSpace(pcfg.Endpoint)
+	if addr == "" {
+		return nil, nil, errBlankPrometheusAddress
+	}
+
+	opts := prometheus.Options{
+		Namespace:   pcfg.Namespace,
+		ConstLabels: pcfg.ConstLabels,
+	}
+	pe, err := prometheus.New(opts)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	ln, err := net.Listen("tcp", addr)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	// The Prometheus metrics exporter has to run on the provided address
+	// as a server that'll be scraped by Prometheus.
+	mux := http.NewServeMux()
+	mux.Handle("/metrics", pe)
+
+	srv := &http.Server{Handler: mux}
+	go func() {
+		_ = srv.Serve(ln)
+	}()
+
+	pexp := &prometheusExporter{exporter: pe}
+
+	return pexp, ln.Close, nil
+}

exporter/prometheusexporter/factory_test.go

@@ -0,0 +1,79 @@
+// Copyright 2019, OpenTelemetry 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 prometheusexporter
+
+import (
+	"testing"
+
+	"github.com/open-telemetry/opentelemetry-service/models"
+
+	"go.uber.org/zap"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/open-telemetry/opentelemetry-service/factories"
+)
+
+func TestCreateDefaultConfig(t *testing.T) {
+	factory := factories.GetExporterFactory(typeStr)
+	require.NotNil(t, factory)
+
+	cfg := factory.CreateDefaultConfig()
+	assert.NotNil(t, cfg, "failed to create default config")
+}
+
+func TestCreateTraceExporter(t *testing.T) {
+	factory := factories.GetExporterFactory(typeStr)
+	cfg := factory.CreateDefaultConfig()
+
+	_, _, err := factory.CreateTraceExporter(zap.NewNop(), cfg)
+	assert.Error(t, err, models.ErrDataTypeIsNotSupported)
+}
+
+func TestCreateMetricsExporter(t *testing.T) {
+	const defaultTestEndPoint = "127.0.0.1:55678"
+	tests := []struct {
+		name     string
+		config   ConfigV2
+		mustFail bool
+	}{
+		{
+			name: "NoEndpoint",
+			config: ConfigV2{
+				Endpoint: "",
+			},
+			mustFail: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			factory := factories.GetExporterFactory(typeStr)
+			consumer, stopFunc, err := factory.CreateMetricsExporter(zap.NewNop(), &tt.config)
+
+			if tt.mustFail {
+				assert.NotNil(t, err)
+			} else {
+				assert.Nil(t, err)
+				assert.NotNil(t, consumer)
+				assert.NotNil(t, stopFunc)
+
+				err = stopFunc()
+				assert.Nil(t, err)
+			}
+		})
+	}
+}