Fixes #191: crash on empty metric name

Signed-off-by: Ivan Izaguirre <ivanizag@gmail.com>

exporter.go

@@ -280,7 +280,7 @@ type Exporter struct {
 
 func escapeMetricName(metricName string) string {
 	// If a metric starts with a digit, prepend an underscore.
-	if metricName[0] >= '0' && metricName[0] <= '9' {
+	if len(metricName) > 0 && metricName[0] >= '0' && metricName[0] <= '9' {
 		metricName = "_" + metricName
 	}
 
@@ -333,6 +333,10 @@ func (b *Exporter) handleEvent(event Event) {
 	metricName := ""
 	prometheusLabels := event.Labels()
 	if present {
+		if mapping.Name == "" {
+			log.Debugf("The mapping for match \"%v\" generates an empty metric name.", mapping.Match)
+			return
+		}
 		metricName = escapeMetricName(mapping.Name)
 		for label, value := range labels {
 			prometheusLabels[label] = value

exporter_test.go

@@ -56,6 +56,38 @@ func TestNegativeCounter(t *testing.T) {
 	ex.Listen(events)
 }
 
+// TestEmptyStringMetric validates when a metric name ends up
+// being the empty string after applying the match replacements
+// tha we don't panic the Exporter Listener.
+func TestEmptyStringMetric(t *testing.T) {
+	events := make(chan Events)
+	go func() {
+		c := Events{
+			&CounterEvent{
+				metricName: "foo_bar",
+				value:      1,
+			},
+		}
+		events <- c
+		close(events)
+	}()
+
+	config := `
+mappings:
+- match: .*_bar
+  match_type: regex
+  name: "${1}"
+`
+	testMapper := &mapper.MetricMapper{}
+	err := testMapper.InitFromYAMLString(config)
+	if err != nil {
+		t.Fatalf("Config load error: %s %s", config, err)
+	}
+
+	ex := NewExporter(testMapper)
+	ex.Listen(events)
+}
+
 // TestInvalidUtf8InDatadogTagValue validates robustness of exporter listener
 // against datadog tags with invalid tag values.
 // It sends the same tags first with a valid value, then with an invalid one.
@@ -167,6 +199,7 @@ func TestEscapeMetricName(t *testing.T) {
 		"with😱emoji":              "with_emoji",
 		"with.*.multiple":         "with___multiple",
 		"test.web-server.foo.bar": "test_web_server_foo_bar",
+		"": "",
 	}
 
 	for in, want := range scenarios {