Feat: Improve Prometheus Metrics

[dennypradipta]

Monika Pull Request (PR)

What feature/issue does this PR add

This PR clears up #1335 and #1336

How did you implement / how did you fix it

I made some new and improve existing exported metrics for Prometheus. The details are below:

Metric Name	Type	Purpose	Labels
monika_alerts_triggered	Counter	Collect count of alerts triggered by a probe	id, name, url, method, alertQuery
monika_alerts_triggered_total	Counter	Collect total count of alerts triggered	-
monika_probes_running	Gauge	Indicates whether a probe is running (1) or idle (0)	id
monika_probes_running_total	Gauge	Collect total count of probes currently running checks	-
monika_probes_status	Gauge	Indicates the current status of a probe: 0 = DOWN, 1 = UP	id, name, url, method
monika_probes_total	Gauge	Collect total number of probes configured	-
monika_request_response_size_bytes	Gauge	Collect size of the response in bytes	id, name, url, method, statusCode, result
monika_request_response_time_seconds	Histogram	Collect duration of probe request in seconds	id, name, url, method, statusCode, result
monika_request_status_code_info	Gauge	Collect HTTP status code of the probe request	id, name, url, method

How to test

1. Use this config

probes:
  - id: 'mock-1'
    name: 'local-test-test'
    requests:
      - url: http://127.0.0.1:7000
    alerts:
      - assertion: response.status < 200 or response.status > 308
        message: HTTP Status is not 200

  - id: 'http-1'
    name: 'status-200-test'
    requests:
      - url: https://httpbin.org/status/200
        method: GET
    alerts:
      - assertion: response.status < 200 or response.status > 308
        message: HTTP Status is not 200
      - assertion: response.time > 2000
        message: Too slow
notifications:
  - id: '1'
    type: 'desktop'

2. Run monika using npm run start -- --prometheus 9090
3. Open localhost:9090/metrics in browser

[dennypradipta]

I've made some changes to the metrics, the docs are also updated.

I've tested this by creating a Grafana dashboard and see how it would look:

You can try to connect your Prometheus to Grafana though, I made a simple repo here:
https://github.com/dennypradipta/monika-dashboard