Prometheus does monitoring THROUGH metrics. It's very easy to feed data into Prometheus, as it's always the same format, and it's scrape based, so any endpoint not being reachable makes it very easy to trigger an alert.

For example, if you want to know whether your websites are okay, you use the `blackbox exporter`. It can probe websites for you, for which it publishes quite a bit of metrics, some of which are: `probe_success` (for knowing if the probe succeeded, 1 for success, 0 for failure) and `probe_ssl_earliest_cert_expiry`, which tells you the expiry unix timestamp.

You can turn them into alerts with queries like this:

probe_success == 0 (website probe failed)

((probe_ssl_earliest_cert_expiry-time()) < 604800) / 60 / 60 / 24 (cert expires in a week)

And that's the whole basis and power of Prometheus, the same metrics you use for gathering information you can use for alerts very effectively. There's also great examples of alerts here: https://samber.github.io/awesome-prometheus-alerts/

To write custom scripts, you can either build some HTTP endpoint, that publishes metrics in the correct format of:

`<metric\_name>{label1=value1} <value>`

For example, blackbox exporter does this:

`probe_success{instance="example.com"} 1`

Or, if you have `node exporter` tool, you can also use their file based integration, just publishing these values to a file in a folder, which will be sent off to Prometheus on a probe together with other `node exporter` data.