Add http endpoint support

[mblaschke]

Add support for custom endpoint (IP/Port or full hostname) to make probes possible in specific and complex environments and to test the service chain:

• support for checking services in a service chain (app gateway -> loadbalancer -> ingress controller -> app) to test http response of eg. the kubernetes ingress (from inside the cluster by using xxx.xxx.svc.cluster.local as endpoint where you cannot reach the loadbalancer due to cloud limitations) and get the ability to setup alerts on probe failure and TLS certificate expiry
• support for checking of "hidden" services which are not directly reachable (eg where there is a NAT in between cloud and on premise and DNS resolves to on premise IPs)
• easier use of prometheus-operator for such checks/environments

example module configuration:

  http_2xx_example_with_endpoint:
    prober: http
    timeout: 5s
    http:
      endpoint: "1.1.1.1:443"
      valid_http_versions: ["HTTP/1.1", "HTTP/2.0"]
      valid_status_codes: []  # Defaults to 2xx
      method: GET
      preferred_ip_protocol: "ip4" # defaults to "ip6"
      ip_protocol_fallback: false  # no fallback to "ip6"

  http_2xx_ingress_internal:
    prober: http
    timeout: 5s
    http:
      endpoint: "controller.ingress.svc.cluster.local:80"
      valid_http_versions: ["HTTP/1.1", "HTTP/2.0"]
      valid_status_codes: []  # Defaults to 2xx
      method: GET
      preferred_ip_protocol: "ip4" # defaults to "ip6"
      ip_protocol_fallback: false  # no fallback to "ip6"

[roidelapluie]

Should we instead have an extra HTTP parameter? &target=1.1.1.1&hostname=example.com ? WDYT?

[2mf]

I really would love the hostname parameter, since I need to probe internal as well as external IPs. I can't not use DNS since I need to test specific endpoints for specific hostnames.

[roidelapluie]

instead of endpoint, you can use Headers: Host in the current blackbox exporter.

I have a design doc to enable this per probe: https://docs.google.com/document/d/1VwqXi2TOb5KXaZY6Iio7411x64pJao3GusX8MqYsJ2g/edit

[mblaschke]

The extra hostname parameter would be also ok. I just want to avoid creating one configuration entry for only one probe.

[roidelapluie]

Okay, let's focus on #823 .

[roidelapluie]

#823 is merged.