Make prometheus reciever config loading strict (#697)

Fixes #583

Using strict umarshalling ensures eliminates common mistakes
such as adding a key intended for the prometheus reciever itself
into the prometheus subconfiguration.

Author: toumorokoshi

receiver/prometheusreceiver/config_test.go

@@ -93,3 +93,20 @@ func TestLoadConfigFailsOnUnknownSection(t *testing.T) {
 	require.Error(t, err)
 	require.Nil(t, cfg)
 }
+
+// As one of the config parameters is consuming prometheus
+// configuration as a subkey, ensure that invalid configuration
+// within the subkey will also raise an error.
+func TestLoadConfigFailsOnUnknownPrometheusSection(t *testing.T) {
+	factories, err := config.ExampleComponents()
+	assert.Nil(t, err)
+
+	factory := &Factory{}
+	factories.Receivers[typeStr] = factory
+	cfg, err := config.LoadConfigFile(
+		t,
+		path.Join(".", "testdata", "invalid-config-prometheus-section.yaml"), factories)
+
+	require.Error(t, err)
+	require.Nil(t, cfg)
+}

receiver/prometheusreceiver/factory.go

@@ -89,7 +89,7 @@ func CustomUnmarshalerFunc(v *viper.Viper, viperKey string, sourceViperSection *
 	out = []byte(os.ExpandEnv(string(out)))
 	config := intoCfg.(*Config)
 
-	err = yaml.Unmarshal(out, &config.PrometheusConfig)
+	err = yaml.UnmarshalStrict(out, &config.PrometheusConfig)
 	if err != nil {
 		return fmt.Errorf("prometheus receiver failed to unmarshal yaml to prometheus config: %s", err)
 	}

receiver/prometheusreceiver/testdata/invalid-config-prometheus-section.yaml

@@ -0,0 +1,20 @@
+receivers:
+  prometheus:
+    config:
+      use_start_time_metric: true
+      scrape_configs:
+        - job_name: 'demo'
+          scrape_interval: 5s
+
+processors:
+  exampleprocessor:
+
+exporters:
+  exampleexporter:
+
+service:
+  pipelines:
+    traces:
+      receivers: [prometheus]
+      processors: [exampleprocessor]
+      exporters: [exampleexporter]