[#150] Add dropped logs metric

Signed-off-by: Pavel Pogodaev <p.pogodaev@yadro.com>

metrics/desc.go

@@ -76,6 +76,15 @@ var appMetricsDesc = map[string]map[string]Description{
 			VariableLabels: []string{"endpoint"},
 		},
 	},
+	statisticSubsystem: {
+		droppedLogs: Description{
+			Type:      dto.MetricType_COUNTER,
+			Namespace: namespace,
+			Subsystem: statisticSubsystem,
+			Name:      droppedLogs,
+			Help:      "Dropped logs (by sampling) count",
+		},
+	},
 }
 
 type Description struct {
@@ -148,3 +157,12 @@ func mustNewGaugeVec(description Description) *prometheus.GaugeVec {
 		description.VariableLabels,
 	)
 }
+
+func mustNewCounter(description Description) prometheus.Counter {
+	if description.Type != dto.MetricType_COUNTER {
+		panic("invalid metric type")
+	}
+	return prometheus.NewCounter(
+		prometheus.CounterOpts(newOpts(description)),
+	)
+}