Initial values for metrics should be supported

[NaomiLYJ]

We tried out the library, here's our sample code

import express from 'express'
import * as prom from 'prom-client';
import moment from 'moment';

// Initialize registry
const promRegistry = new prom.Registry();
const defaultLabels = { machineId: 'test-pc', appName: 'nodejs-app' };
promRegistry.setDefaultLabels(defaultLabels);

// Initialize a Counter
const httpRequests = new prom.Counter({
  name: 'http_requests',
  help: 'Count of successful requests',
  labelNames: ['endpoint', 'status'],
  registers: [promRegistry] // specify a non-default registry
});

// Instantiate the Counter with specific labels
const helloWorldHttpSuccess = httpRequests.labels({ "endpoint": "hello-world", "status": "2xx" })

// Initialize a Summary
const httpRequestsProcessTimesSummary = new prom.Summary({
  name: 'http_requests_process_times',
  help: 'How long does it take to process the request',
  labelNames: ['endpoint'],
  maxAgeSeconds: 60,
  ageBuckets: 6,
  percentiles: [0, 0.5, 0.95, 0.99, 1],
  registers: [promRegistry] // specify a non-default registry
});

// Instantiate the Sumary with specific labels
const helloWorldHttpProcessTimesSummary = httpRequestsProcessTimesSummary.labels({ "endpoint": "hello-world" })

// Expose metrics in prometheus format
// Listening on port 9090 for metrics
const prometheus = express();
const prometheusPort = 8090;
prometheus.get('/metrics', async (req, res) => {
  res.type('text/plain');
  res.send(await promRegistry.metrics())
})
prometheus.listen(prometheusPort, () => {
  console.log(`Prometheus metrics listening on port ${prometheusPort}`)
})

Current Behavior

The metrics endpoint output does not show the metrics which are not triggered even though they are already instantiated with labels, e.g.,

# HELP http_requests Count of successful requests
# TYPE http_requests counter

# HELP http_requests_process_times How long does it take to process the request
# TYPE http_requests_process_times summary

Expected

The metrics endpoint outputs initial values (e.g., 0 for a counter) for instantiated metrics

# HELP http_requests Count of successful requests
# TYPE http_requests counter
http_requests{endpoint="hello-world",status="2xx",machineId="test-pc",appName="nodejs-app"} 0

# HELP http_requests_process_times How long does it take to process the request
# TYPE http_requests_process_times summary
http_requests_process_times{quantile="0",endpoint="hello-world",machineId="test-pc",appName="nodejs-app"} NaN
http_requests_process_times{quantile="0.5",endpoint="hello-world",machineId="test-pc",appName="nodejs-app"} NaN
http_requests_process_times{quantile="0.95",endpoint="hello-world",machineId="test-pc",appName="nodejs-app"} NaN
http_requests_process_times{quantile="0.99",endpoint="hello-world",machineId="test-pc",appName="nodejs-app"} NaN
http_requests_process_times{quantile="1",endpoint="hello-world",machineId="test-pc",appName="nodejs-app"} NaN
http_requests_process_times_sum{endpoint="hello-world",machineId="test-pc",appName="nodejs-app"} 0
http_requests_process_times_count{endpoint="hello-world",machineId="test-pc",appName="nodejs-app"} 0

The initial values make sure that the defined metrics always show up at the metrics output, which would avoid a lot of issues (e.g., data not found) when used together with Grafana for visualization.

The initial values are supported in other languages such as C++, Golang, Python etc..

[zbjornson]

#438 (comment) has a suggested fix for this and background documentation: call inc(0). I think that's the best option, as this library can't generally know the full set of label values.

Histograms additionally have a .zero() method, and summaries are maybe supposed to have one also (#481).

The initial values are supported in other languages such as C++, Golang, Python etc..

I looked through the golang and python libraries and didn't see a specific initialization method. If inc(0) doesn't work for you, can you point me to the specific methods there please?

[NaomiLYJ]

Thanks a lot @zbjornson for your prompt reply!

Not sure if I explained it properly. The initialization here I meant is that initial values are provided once we instantiated the metric with labels, e.g.

// Instantiate the Counter with specific labels
const helloWorldHttpSuccess = httpRequests.labels({ "endpoint": "hello-world", "status": "2xx" })

We do not need to call extra functions afterwards. This feature is implemented in C++, Golang and Python.

The way you proposed is working, but probably not clean. It would be great if this feature could be implemented.