Make Prometheus Collector Filter Custom Metrics

Previously, the Prometheus collector ignored the
`MetricsConfig` field of the custom metrics specification,
simply storing all exposed metrics.  Now, if the `MetricsConfig`
field contains any metric names, only those metrics will be stored
and exposed.

Fixes google#1005

collector/config/sample_config_prometheus_filtered.json

@@ -0,0 +1,8 @@
+{
+        "endpoint" : "http://localhost:8080/metrics",
+	"polling_frequency" : 10, 
+        "metrics_config" : [
+            "go_goroutines"
+        ]
+}
+

collector/prometheus_collector.go

@@ -35,6 +35,9 @@ type PrometheusCollector struct {
 
 	//holds information extracted from the config file for a collector
 	configFile Prometheus
+
+	// the metrics to gather (uses a map as a set)
+	metricsSet map[string]bool
 }
 
 //Returns a new collector using the information extracted from the configfile
@@ -54,11 +57,20 @@ func NewPrometheusCollector(collectorName string, configFile []byte) (*Prometheu
 		minPollingFrequency = minSupportedFrequency
 	}
 
+	var metricsSet map[string]bool
+	if len(configInJSON.MetricsConfig) > 0 {
+		metricsSet = make(map[string]bool, len(configInJSON.MetricsConfig))
+		for _, name := range configInJSON.MetricsConfig {
+			metricsSet[name] = true
+		}
+	}
+
 	//TODO : Add checks for validity of config file (eg : Accurate JSON fields)
 	return &PrometheusCollector{
 		name:             collectorName,
 		pollingFrequency: minPollingFrequency,
 		configFile:       configInJSON,
+		metricsSet:       metricsSet,
 	}, nil
 }
 
@@ -100,8 +112,12 @@ func (collector *PrometheusCollector) GetSpec() []v1.MetricSpec {
 			if stopIndex == -1 {
 				stopIndex = strings.Index(lines[i+2], " ")
 			}
+			name := strings.TrimSpace(lines[i+2][0:stopIndex])
+			if _, ok := collector.metricsSet[name]; collector.metricsSet != nil && !ok {
+				continue
+			}
 			spec := v1.MetricSpec{
-				Name:   strings.TrimSpace(lines[i+2][0:stopIndex]),
+				Name:   name,
 				Type:   v1.MetricType(getMetricData(lines[i+1])),
 				Format: "float",
 				Units:  getMetricData(lines[i]),
@@ -145,6 +161,9 @@ func (collector *PrometheusCollector) Collect(metrics map[string][]v1.MetricVal)
 			}
 
 			metName := strings.TrimSpace(line[0:startLabelIndex])
+			if _, ok := collector.metricsSet[metName]; collector.metricsSet != nil && !ok {
+				continue
+			}
 
 			if startLabelIndex+1 <= spaceIndex-1 {
 				metLabel = strings.TrimSpace(line[(startLabelIndex + 1):(spaceIndex - 1)])

collector/prometheus_collector_test.go

@@ -63,3 +63,38 @@ func TestPrometheus(t *testing.T) {
 	goRoutines := metrics["go_goroutines"]
 	assert.Equal(goRoutines[0].FloatValue, 16)
 }
+
+func TestPrometheusFiltersMetrics(t *testing.T) {
+	assert := assert.New(t)
+
+	//Create a prometheus collector using the config file 'sample_config_prometheus_filtered.json'
+	configFile, err := ioutil.ReadFile("config/sample_config_prometheus_filtered.json")
+	collector, err := NewPrometheusCollector("Prometheus", configFile)
+	assert.NoError(err)
+	assert.Equal(collector.name, "Prometheus")
+	assert.Equal(collector.configFile.Endpoint, "http://localhost:8080/metrics")
+
+	tempServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+
+		text := "# HELP go_gc_duration_seconds A summary of the GC invocation durations.\n"
+		text += "# TYPE go_gc_duration_seconds summary\n"
+		text += "go_gc_duration_seconds{quantile=\"0\"} 5.8348000000000004e-05\n"
+		text += "go_gc_duration_seconds{quantile=\"1\"} 0.000499764\n"
+		text += "# HELP go_goroutines Number of goroutines that currently exist.\n"
+		text += "# TYPE go_goroutines gauge\n"
+		text += "go_goroutines 16"
+		fmt.Fprintln(w, text)
+	}))
+
+	defer tempServer.Close()
+
+	collector.configFile.Endpoint = tempServer.URL
+	metrics := map[string][]v1.MetricVal{}
+	_, metrics, errMetric := collector.Collect(metrics)
+
+	assert.NoError(errMetric)
+	assert.Len(metrics, 1)
+
+	goRoutines := metrics["go_goroutines"]
+	assert.Equal(goRoutines[0].FloatValue, 16)
+}