Values from observable gauge is not pushed

[pzur1]

Hi i have server like this:
`const app = require('express')();
const http = require('http').Server(app);
const io = require('socket.io')(http);
const port = process.env.PORT || 3000;

//instrumentation
const { NodeSDK } = require('@opentelemetry/sdk-node');
const { ConsoleSpanExporter } = require('@opentelemetry/sdk-trace-node');

//prometheus exporter
const opentelemetry = require('@opentelemetry/sdk-node');
const {
getNodeAutoInstrumentations,
} = require('@opentelemetry/auto-instrumentations-node');
const { PrometheusExporter } = require('@opentelemetry/exporter-prometheus');
const { PeriodicExportingMetricReader } = require('@opentelemetry/sdk-metrics');
const { Resource } = require('@opentelemetry/resources');
const { metrics } = require('@opentelemetry/api');
const { OTLPMetricExporter } = require('@opentelemetry/exporter-metrics-otlp-grpc');
const { MeterProvider } = require('@opentelemetry/sdk-metrics-base');
const { HostMetrics } = require('@opentelemetry/host-metrics');

//const memoryData = process.memoryUsage();
//const cpuData = process.cpuUsage();
//const data = process.resourceUsage

const exporter = new PrometheusExporter(
{
startServer: true,
port : 56326
}, () => {
console.log('prometheus scrape endpoint: http://localhost:56326/metrics')
}
)
const meterProvider = new MeterProvider()
meterProvider.addMetricReader(exporter)

var counter = meterProvider.getMeter('prometheus').createCounter("failed_requests_metric", {
description: "Indicates failed requests",
monotonic: true,
labelKeys: ["failed_requests_metric"],
});

var timeOfProcessing =0;
var time_of_processing_observable_gauge = meterProvider.getMeter('prometheus').createObservableGauge("time_of_processing", {
description: "sum of all messages processing time",
//monotonic: true,
labelKeys: ["time_of_processing"],
});
time_of_processing_observable_gauge.addCallback(()=> timeOfProcessing)

var processed = 0;
var counterOfProcessed = meterProvider.getMeter('prometheus').createCounter("counter_of_processed", {
description: "all processed messages",
monotonic: true,
labelKeys: ["counter_of_processed"],
});
//counterOfProcessed_observable_gauge.addCallback(()=> processed)

var timeBetweenReceiveAndSend;
var time_between_receive_and_send_observable_gauge = meterProvider.getMeter('prometheus').createObservableGauge("time_between_receive_and_send", {
description: "sum of all messages receive plus send time",
monotonic: true,
labelKeys: ["time_between_receive_and_send"],
});
time_between_receive_and_send_observable_gauge.addCallback(()=> timeBetweenReceiveAndSend);

let timeOfTransport;
var time_of_transport_observable_gauge = meterProvider.getMeter('prometheus').createObservableGauge("time_of_transport", {
description: "sum of all messages transport (from client to server) time",
monotonic: true,
labelKeys: ["time_of_transport"],
});
time_of_transport_observable_gauge.addCallback(()=>timeOfTransport);

const hostMetrics = new HostMetrics({ meterProvider, name: 'example-host-metrics' })
hostMetrics.start();

function method1()
{
processed+=1;
}

try{
app.get('/', (req, res) => {
res.sendFile(__dirname + '/index.html');
});

io.on('connection', (socket) => {
socket.on('chat message', msg => {

var currentTime = Date.now();

var splitted = msg.split("//");
var time = splitted[1];
var message = splitted[0];
var timeOfTransportLocal = currentTime - time;

counterOfProcessed.add(1);
//this.processed+=1;
timeOfTransport+=timeOfTransportLocal;
//method1();
var timeStart = Date.now();
io.emit('chat message', msg);//Object.keys(io.eio.clients));
var timeEnd = Date.now();
timeOfProcessing+=timeEnd-timeStart;

});
});

http.listen(port, () => {
console.log(Socket.IO server running at http://localhost:${port}/);
});
}
catch(e){
counter.add(1);
throw e;
}`

Inside socket.on I adding to counterOfProcess and it works - i see `# HELP counter_of_processed_total all processed messages

TYPE counter_of_processed_total counter

counter_of_processed_total 1` in prometheus

But if it comes to updating value timeOfProcessing+=timeEnd-timeStart it doesn't work. I see on debug that value is set (in callback also!) but in prometheus i see:
`# HELP time_of_processing sum of all messages processing time

TYPE time_of_processing gauge`

It's quie weird. Please for help