Reference for code :

https://github.com/RitikaxG/Promethus-Metrics-Counter-Guage-Histogram-

Prometheus is a self-hosted monitoring system that:

• Scrapes metrics from applications or exporters by calling their /metrics HTTP endpoints.
• Uses a pull model (Prometheus pulls data, apps don’t push).
• Stores all data in its own time-series database.
• Lets you query that data using PromQL (for graphs, alerts, dashboards).
• Sends alerts via Alert manager.
• Typically pairs with Grafana for visualisation.

In short: Prometheus = metrics scraper + time-series DB + alerting engine (pull-based monitoring).

Architecture

• Prometheus can’t be scaled horizontally

  Can’t create multiple machines that’s distributing load among themselves.

• Prometheus store data in time series format

  Even if no one’s hitting ur application, u re still collecting metrics every x seconds

Write logic for Logging Response time for every request

import express from "express"
import type { Request, Response, NextFunction } from "express";
const app = express();

app.use(express.json())

const middleware = (req : Request,res : Response, next : NextFunction) => {
    const startTime = Date.now();
    
    res.on("finish",()=>{
        const endTime = Date.now();
        console.log(`Time taken : ${endTime - startTime}ms | Method Type : ${req.method} | Request route : ${req.path}`)

    })

    next()
}

// Every endpoint will use middleware 
app.use(middleware);

app.get("/cpu",(req,res) => {
    let sum = 0
    for(let i=0;i<100000000;i++){
        sum += i
    }
    return res.json({
        message : "Expensive operation successfully completed",
        sum 
    })
})

app.get("/health",(req,res) => {
    return res.send("ok")
})

app.listen(3000,()=>{
    console.log("Listening on port 3000")
})

Type of Metrics

• Counter ( Metrics that only increase with time. Eg. No of request hitting your server