RFE: Support explicit failure in tcp prober query_response

[siebenmann]

In a variety of protocols, such as SMTP, there are server responses that indicate that your probe has definitely failed. At the moment, tcp prober query_response things cannot match these and abort; instead, the server feeds you text, the text fails to match your positive expect:, and eventually things time out. It would be useful if query_response items could also contain an explicit reject: (as a hypothetical name) that would cause the probe to immediately fail if they matched.

A hypothetical example:

  smtp_starttls:
    prober: tcp
    tcp:
      query_response:
        - expect: "^220 "
          reject: "^[45]"
        - send: "EHLO test.example.org\r"
        - expect: "^250-STARTTLS"
          reject: "^[45]"
        - expect: "^250 "
        - send: "STARTTLS\r"
        - expect: "^220"
        - starttls: true
        - send: "EHLO test.example.org\r"
        - expect: "^250 "
        - send: "MAIL FROM:<test-addr@test.example.org>\r"
        - expect: "^250 "
          reject: "^[45]"
        - send: "QUIT\r"

If this is added, it would be ideal to add a probe_ metric that indicated that the probe had failed because it matched an explicit reject:, instead of timing out or something else going on.