chore(storage/remote): collect maxTimestamp when value is 0 as well.

This change enables the PrometheusRemoteWriteBehind alert’s expression to be evaluated
even when the remote endpoint has never been reached. As a result, PrometheusRemoteWriteBehind
will fire to easily detect configuration mistakes (such as incorrect endpoint URLs) or
unrecoverable connectivity issues.

See https://github.com/prometheus/prometheus/issues/14350 for details.

Signed-off-by: machine424 <ayoubmrini424@gmail.com>

storage/remote/max_timestamp.go

@@ -39,9 +39,3 @@ func (m *maxTimestamp) Get() float64 {
 	defer m.mtx.Unlock()
 	return m.value
 }
-
-func (m *maxTimestamp) Collect(c chan<- prometheus.Metric) {
-	if m.Get() > 0 {
-		m.Gauge.Collect(c)
-	}
-}

storage/remote/queue_manager.go

@@ -232,7 +232,7 @@ func newQueueManagerMetrics(r prometheus.Registerer, rn, e string) *queueManager
 			Namespace:   namespace,
 			Subsystem:   subsystem,
 			Name:        "queue_highest_sent_timestamp_seconds",
-			Help:        "Timestamp from a WAL sample, the highest timestamp successfully sent by this queue, in seconds since epoch.",
+			Help:        "Timestamp from a WAL sample, the highest timestamp successfully sent by this queue, in seconds since epoch. Initialized to 0 when no data has been sent yet.",
 			ConstLabels: constLabels,
 		}),
 	}

storage/remote/queue_manager_test.go

@@ -60,7 +60,7 @@ func newHighestTimestampMetric() *maxTimestamp {
 			Namespace: namespace,
 			Subsystem: subsystem,
 			Name:      "highest_timestamp_in_seconds",
-			Help:      "Highest timestamp that has come into the remote storage via the Appender interface, in seconds since epoch.",
+			Help:      "Highest timestamp that has come into the remote storage via the Appender interface, in seconds since epoch. Initialized to 0 when no data has been received yet",
 		}),
 	}
 }

storage/remote/write.go

@@ -100,7 +100,7 @@ func NewWriteStorage(logger log.Logger, reg prometheus.Registerer, dir string, f
 				Namespace: namespace,
 				Subsystem: subsystem,
 				Name:      "highest_timestamp_in_seconds",
-				Help:      "Highest timestamp that has come into the remote storage via the Appender interface, in seconds since epoch.",
+				Help:      "Highest timestamp that has come into the remote storage via the Appender interface, in seconds since epoch. Initialized to 0 when no data has been received yet.",
 			}),
 		},
 	}