Monitor-Web | come across a website | Browser Plugin library

You can use Multi-step Web Tests for this:

https://docs.microsoft.com/en-us/azure/application-insights/app-insights-monitor-web-app-availability#multi-step-web-tests

I think current pricing is $10/month (all locations are included).

EDIT: Simple Web Tests are free.

It would take to long if I was to explain all parts that can be done, so I'll keep it short.

Usually you define all these details in a Service Level Agreement where you also define the availability target (i.e. 99 %) that also include planned downtime. A 99 % availability target is to have the app running and its functionality as described in the document with at most approx. 87.6 h per year. Here is a SLA uptime calculator.

The normal interval is 5 minutes as you say, but it you can prove by using an external site / service that the suppliers are not meeting the requirements, you calculate your loss (revenue loss, labor costs etc) and claim the money from them. You already have a Business Impact Analysis (BIA) I guess otherwise you should do it.

Ok, now to the programming / DevOps part. I usually develop applications / services with this in mind and report its status to a third party service like NewRelic, Uptrends or similar. As an example I also use a self-made service for this because accurate requirements for delivering data at least once a second with a hard deadline. In my solution I use WebSockets to send data in both directions following a schedule, event or when needed. A benefit with that is that you can send status (good or bad) let say every 500 ms and you will know within one second if the app has failed (≈ 499 ms + 500 ms).

Using a service like this you can measure the uptime, custom events of interest and possible errors within a second and a ton of other metrics. Usually within 5-100 ms but WCET/WCRT is hard to estimate.

To answer your question, you cannot calculate application availability with so few measure points, once every 5 min is covering approx. 12 seconds per hour and you cannot have any reliable calculation from that. You can assume everything was ok between the measure points but that is called guessing. I have made implementations that have 14 400 measure points per hour in order to provide 500 ms accuracy (Banks).

I hope you got an answer that helps you with your problem.

Passing the basic auth credentials in the URL has been deprecated by RFC 3986 (Here is a snippet from the RFC)

3.2.1. User Information

The userinfo subcomponent may consist of a user name and, optionally, scheme-specific information about how to gain authorization to access the resource. The user information, if present, is followed by a commercial at-sign ("@") that delimits it from the host.

UPDATED ANSWER:

The Performance section lists how long it took from your server receiving the request to sending something back to the client, it doesn't count network latency at all.

I believe they only check the response status without downloading the content if you don't require a content match.

Below is an example of the configuration for my blog.

If you wish, you can make sure the test downloads the content by ticking the Content match checkbox, and specifying that the content must contain the text that is somewhere near the end of your index page (like in a footer).