pkg/promtail: Initialize counters to 0 when creating client

This ensures metrics are not missing before they're incremented the first
time, as recommended by the upstream Prometheus instrumentation guidelines:
https://prometheus.io/docs/practices/instrumentation/#avoid-missing-metrics.

Currently, in the case of dropped_entries_total and dropped_bytes_total,
the metric is simply missing until the first occurrence of an entry
being dropped.

pkg/promtail/client/client.go

@@ -65,6 +65,10 @@ var (
 		Name:      "request_duration_seconds",
 		Help:      "Duration of send requests.",
 	}, []string{"status_code", "host"})
+
+	countersWithHost = []*prometheus.CounterVec{
+		encodedBytes, sentBytes, droppedBytes, sentEntries, droppedEntries,
+	}
 )
 
 func init() {
@@ -129,6 +133,12 @@ func New(cfg Config, logger log.Logger) (Client, error) {
 
 	c.client.Timeout = cfg.Timeout
 
+	// Initialize counters to 0 so the metrics are exported before the first
+	// occurrence of incrementing to avoid missing metrics.
+	for _, counter := range countersWithHost {
+		counter.WithLabelValues(c.cfg.URL.Host).Add(0)
+	}
+
 	c.wg.Add(1)
 	go c.run()
 	return c, nil

pkg/promtail/client/client_test.go

@@ -70,6 +70,9 @@ func TestClient_Handle(t *testing.T) {
 				# HELP promtail_sent_entries_total Number of log entries sent to the ingester.
 				# TYPE promtail_sent_entries_total counter
 				promtail_sent_entries_total{host="__HOST__"} 3.0
+				# HELP promtail_dropped_entries_total Number of log entries dropped because failed to be sent to the ingester after all retries.
+				# TYPE promtail_dropped_entries_total counter
+				promtail_dropped_entries_total{host="__HOST__"} 0
 			`,
 		},
 		"batch log entries together until the batch wait time is reached": {
@@ -93,6 +96,9 @@ func TestClient_Handle(t *testing.T) {
 				# HELP promtail_sent_entries_total Number of log entries sent to the ingester.
 				# TYPE promtail_sent_entries_total counter
 				promtail_sent_entries_total{host="__HOST__"} 2.0
+				# HELP promtail_dropped_entries_total Number of log entries dropped because failed to be sent to the ingester after all retries.
+				# TYPE promtail_dropped_entries_total counter
+				promtail_dropped_entries_total{host="__HOST__"} 0
 			`,
 		},
 		"retry send a batch up to backoff's max retries in case the server responds with a 5xx": {
@@ -119,6 +125,9 @@ func TestClient_Handle(t *testing.T) {
 				# HELP promtail_dropped_entries_total Number of log entries dropped because failed to be sent to the ingester after all retries.
 				# TYPE promtail_dropped_entries_total counter
 				promtail_dropped_entries_total{host="__HOST__"} 1.0
+				# HELP promtail_sent_entries_total Number of log entries sent to the ingester.
+				# TYPE promtail_sent_entries_total counter
+				promtail_sent_entries_total{host="__HOST__"} 0
 			`,
 		},
 		"do not retry send a batch in case the server responds with a 4xx": {
@@ -137,6 +146,9 @@ func TestClient_Handle(t *testing.T) {
 				# HELP promtail_dropped_entries_total Number of log entries dropped because failed to be sent to the ingester after all retries.
 				# TYPE promtail_dropped_entries_total counter
 				promtail_dropped_entries_total{host="__HOST__"} 1.0
+				# HELP promtail_sent_entries_total Number of log entries sent to the ingester.
+				# TYPE promtail_sent_entries_total counter
+				promtail_sent_entries_total{host="__HOST__"} 0
 			`,
 		},
 		"batch log entries together honoring the client tenant ID": {
@@ -156,6 +168,9 @@ func TestClient_Handle(t *testing.T) {
 				# HELP promtail_sent_entries_total Number of log entries sent to the ingester.
 				# TYPE promtail_sent_entries_total counter
 				promtail_sent_entries_total{host="__HOST__"} 2.0
+				# HELP promtail_dropped_entries_total Number of log entries dropped because failed to be sent to the ingester after all retries.
+				# TYPE promtail_dropped_entries_total counter
+				promtail_dropped_entries_total{host="__HOST__"} 0
 			`,
 		},
 		"batch log entries together honoring the tenant ID overridden while processing the pipeline stages": {
@@ -183,6 +198,9 @@ func TestClient_Handle(t *testing.T) {
 				# HELP promtail_sent_entries_total Number of log entries sent to the ingester.
 				# TYPE promtail_sent_entries_total counter
 				promtail_sent_entries_total{host="__HOST__"} 4.0
+				# HELP promtail_dropped_entries_total Number of log entries dropped because failed to be sent to the ingester after all retries.
+				# TYPE promtail_dropped_entries_total counter
+				promtail_dropped_entries_total{host="__HOST__"} 0
 			`,
 		},
 	}