expose latest NATS message sequence number as metric

Author: derhuerst

lib/match.js

@@ -1,6 +1,7 @@
 import {ok} from 'node:assert'
 import {cpus as osCpus} from 'node:os'
 import {
+	Gauge,
 	Counter,
 	Summary,
 } from 'prom-client'
@@ -85,6 +86,11 @@ const runGtfsMatching = async (cfg, opt = {}) => {
 		...opt,
 	}
 
+	const natsMsgSeq = new Gauge({
+		name: 'nats_msg_seq',
+		help: 'sequence number of the latest NATS message being processed',
+		registers: [register],
+	})
 	const successesTotal = new Counter({
 		name: 'matching_successes_total',
 		help: 'number of successfully matched movements/trips',
@@ -176,6 +182,8 @@ const runGtfsMatching = async (cfg, opt = {}) => {
 			seq,
 			dataSlice: data.slice(0, 100).toString('utf8'),
 		}, 'processing AUS IstFahrt msg')
+		natsMsgSeq.set(seq) // todo: is `seq` an integer?
+
 		try {
 			const ausIstFahrt = msg.json(data)
 			// todo: validate against schema, error-log and abort if invalid