Adding metric testing on relay

Signed-off-by: Pedro Tanaka <pedro.tanaka@shopify.com>

pkg/relay/relay_test.go

@@ -1,7 +1,10 @@
 package relay
 
 import (
+	"fmt"
 	"github.com/go-kit/log"
+	"github.com/prometheus/client_golang/prometheus"
+	dto "github.com/prometheus/client_model/go"
 	"github.com/prometheus/statsd_exporter/pkg/clock"
 	"github.com/stvp/go-udp-testing"
 	"testing"
@@ -51,11 +54,97 @@ func TestRelay_RelayLine(t *testing.T) {
 			udp.ShouldReceive(t, tt.args.expected, func() {
 				for _, line := range tt.args.lines {
 					r.RelayLine(line)
-					// Tick time forward to trigger a flush
 				}
+				// Tick time forward to trigger a packet send.
 				clock.ClockInstance.Instant = time.Unix(20000, 0)
 				clock.ClockInstance.TickerCh <- time.Unix(20000, 0)
 			})
+
+			metrics, err := prometheus.DefaultGatherer.Gather()
+			if err != nil {
+				t.Fatalf("Cannot gather from DefaultGatherer: %v", err)
+			}
+
+			metricNames := map[string]float64{
+				"statsd_exporter_relay_long_lines_total": 0,
+			}
+			for metricName, expectedValue := range metricNames {
+				firstMetric := getFloat64(metrics, metricName, prometheus.Labels{"target": "localhost:1160"})
+
+				if firstMetric == nil {
+					t.Fatalf("Could not find time series with first label set for metric: %s", metricName)
+				}
+				if *firstMetric != expectedValue {
+					t.Errorf("Expected metric %s to be %f, got %f", metricName, expectedValue, *firstMetric)
+				}
+			}
 		})
 	}
 }
+
+// getFloat64 search for metric by name in array of MetricFamily and then search a value by labels.
+// Method returns a value or nil if metric is not found.
+func getFloat64(metrics []*dto.MetricFamily, name string, labels prometheus.Labels) *float64 {
+	var metricFamily *dto.MetricFamily
+	for _, m := range metrics {
+		if *m.Name == name {
+			metricFamily = m
+			break
+		}
+	}
+	if metricFamily == nil {
+		return nil
+	}
+
+	var metric *dto.Metric
+	labelStr := fmt.Sprintf("%v", labels)
+	for _, m := range metricFamily.Metric {
+		l := labelPairsAsLabels(m.GetLabel())
+		ls := fmt.Sprintf("%v", l)
+		if labelStr == ls {
+			metric = m
+			break
+		}
+	}
+	if metric == nil {
+		return nil
+	}
+
+	var value float64
+	if metric.Gauge != nil {
+		value = metric.Gauge.GetValue()
+		return &value
+	}
+	if metric.Counter != nil {
+		value = metric.Counter.GetValue()
+		return &value
+	}
+	if metric.Histogram != nil {
+		value = metric.Histogram.GetSampleSum()
+		return &value
+	}
+	if metric.Summary != nil {
+		value = metric.Summary.GetSampleSum()
+		return &value
+	}
+	if metric.Untyped != nil {
+		value = metric.Untyped.GetValue()
+		return &value
+	}
+	panic(fmt.Errorf("collected a non-gauge/counter/histogram/summary/untyped metric: %s", metric))
+}
+
+func labelPairsAsLabels(pairs []*dto.LabelPair) (labels prometheus.Labels) {
+	labels = prometheus.Labels{}
+	for _, pair := range pairs {
+		if pair.Name == nil {
+			continue
+		}
+		value := ""
+		if pair.Value != nil {
+			value = *pair.Value
+		}
+		labels[*pair.Name] = value
+	}
+	return
+}