persist counters and values

example:

const fs = require('fs');
const client = require('prom-client');
const path = './impressions.json';

function loadImpressions() {
  try {
    return JSON.parse(fs.readFileSync(path)).count || 0;
  } catch (err) {
    return 0;
  }
}

function saveImpressions(count) {
  fs.writeFileSync(path, JSON.stringify({ count }));
}

const impressions = new client.Counter({
  name: 'impression_count',
  help: 'Number of impressions',
});

// Load saved state
let persistedCount = loadImpressions();
impressions.inc(persistedCount); // set initial value on startup

// Whenever you track an impression:
function recordImpression() {
  impressions.inc();
  persistedCount++;
  saveImpressions(persistedCount);
}