collectors: distinguish attempts by payment status

Add a label to the metric for tracking htlc attempts so
that we can differentiate between attempts made as part
of payments that are successfull from attempts for payments
which fail.

This allows you to answer questions like: How many attempts
does a successful payment take on average?

Author: calvinrzachman

collectors/payments_collector.go

@@ -23,13 +23,15 @@ var (
 		[]string{"status"},
 	)
 
-	// totalHTLCAttempts is a simple counter which, in combination with the
-	// payment counter, permits tracking the number of attempts per payment.
-	totalHTLCAttempts = prometheus.NewCounter(
+	// totalHTLCAttempts tracks the number of HTLC attempts made based on
+	// the payment status (success or fail). When combined with the payment
+	// counter, this permits tracking the number of attempts per payment.
+	totalHTLCAttempts = prometheus.NewCounterVec(
 		prometheus.CounterOpts{
 			Name: "lnd_total_htlc_attempts",
-			Help: "Total number of HTLC attempts across all payments",
+			Help: "Total number of HTLC attempts across all payments, labeled by final payment status",
 		},
+		[]string{"status"},
 	)
 
 	// paymentAttempts is a histogram for visualizing what portion of
@@ -164,12 +166,13 @@ func processPaymentUpdate(payment *lnrpc.Payment) {
 		status = "unknown"
 	}
 
+	// Increment metrics with proper label.
 	totalPayments.WithLabelValues(status).Inc()
+
 	attemptCount := len(payment.Htlcs)
+	totalHTLCAttempts.WithLabelValues(status).Add(float64(attemptCount))
 
-	totalHTLCAttempts.Add(float64(attemptCount))
 	paymentAttempts.Observe(float64(attemptCount))
-
 	paymentLogger.Debugf("Payment %s updated: status=%s, %d attempts",
 		payment.PaymentHash, status, attemptCount)
 }