Add support Tcp dns probes

[aitorpazos]

This PR tries to address the issue I created myself sometime before: #680

Tested with the following config file:

probe {
  name: "google_com_default"
  type: DNS
  targets {
    host_names: "8.8.8.8"
  }
  interval_msec: 15000
  timeout_msec: 5000
  latency_distribution {
    explicit_buckets: "0.00025,0.0005,0.001,0.002,0.004,0.008,0.016,0.032,0.064,0.128,0.256,0.512,1.024,2.048,4.096,8.192"
  }
  latency_unit: "s"
  dns_probe {
    query_type: A
    resolved_domain: "google.com"
    resolve_first: true
  }
}

probe {
  name: "google_com_udp"
  type: DNS
  targets {
    host_names: "8.8.8.8"
  }
  interval_msec: 15000
  timeout_msec: 5000
  latency_distribution {
    explicit_buckets: "0.00025,0.0005,0.001,0.002,0.004,0.008,0.016,0.032,0.064,0.128,0.256,0.512,1.024,2.048,4.096,8.192"
  }
  additional_label {
    key: "dns_proto"
    value: "udp"
  }
  latency_unit: "s"
  dns_probe {
    query_type: A
    resolved_domain: "google.com"
    resolve_first: true
    dns_proto: "udp"
  }
}

probe {
  name: "google_com_tcp"
  type: DNS
  targets {
    host_names: "8.8.8.8"
  }
  interval_msec: 15000
  timeout_msec: 5000
  latency_distribution {
    explicit_buckets: "0.00025,0.0005,0.001,0.002,0.004,0.008,0.016,0.032,0.064,0.128,0.256,0.512,1.024,2.048,4.096,8.192"
  }
  additional_label {
    key: "dns_proto"
    value: "tcp"
  }
  latency_unit: "s"
  dns_probe {
    query_type: A
    resolved_domain: "google.com"
    resolve_first: true
    dns_proto: "tcp"
  }
}

I can see the different metrics being reported:

...
# TYPE total counter
total{ptype="dns",probe="google_com_default",dst="8.8.8.8"} 1 1707496047071
total{ptype="dns",probe="google_com_udp",dst="8.8.8.8",dns_proto="udp"} 1 1707496049072
total{ptype="dns",probe="google_com_tcp",dst="8.8.8.8",dns_proto="tcp"} 1 1707496051072
# TYPE success counter
success{ptype="dns",probe="google_com_default",dst="8.8.8.8"} 1 1707496047071
success{ptype="dns",probe="google_com_udp",dst="8.8.8.8",dns_proto="udp"} 1 1707496049072
success{ptype="dns",probe="google_com_tcp",dst="8.8.8.8",dns_proto="tcp"} 1 1707496051072
...

Also verified that the configuration is respected through some packets captures:

[manugarg]

Thanks @aitorpazos for this PR.

Please see my comment inline.

[manugarg]

One more comment regarding the test.

[aitorpazos]

With latest commit switching the DNSProto config from string to an Enum, this is the right config file:

probe {
  name: "google_com_default"
  type: DNS
  targets {
    host_names: "8.8.8.8"
  }
  interval_msec: 15000
  timeout_msec: 5000
  latency_distribution {
    explicit_buckets: "0.00025,0.0005,0.001,0.002,0.004,0.008,0.016,0.032,0.064,0.128,0.256,0.512,1.024,2.048,4.096,8.192"
  }
  latency_unit: "s"
  dns_probe {
    query_type: A
    resolved_domain: "google.com"
    resolve_first: true
  }
}

probe {
  name: "google_com_udp"
  type: DNS
  targets {
    host_names: "8.8.8.8"
  }
  interval_msec: 15000
  timeout_msec: 5000
  latency_distribution {
    explicit_buckets: "0.00025,0.0005,0.001,0.002,0.004,0.008,0.016,0.032,0.064,0.128,0.256,0.512,1.024,2.048,4.096,8.192"
  }
  additional_label {
    key: "dns_proto"
    value: "udp"
  }
  latency_unit: "s"
  dns_probe {
    query_type: A
    resolved_domain: "google.com"
    resolve_first: true
    dns_proto: UDP
  }
}

probe {
  name: "google_com_tcp"
  type: DNS
  targets {
    host_names: "8.8.8.8"
  }
  interval_msec: 15000
  timeout_msec: 5000
  latency_distribution {
    explicit_buckets: "0.00025,0.0005,0.001,0.002,0.004,0.008,0.016,0.032,0.064,0.128,0.256,0.512,1.024,2.048,4.096,8.192"
  }
  additional_label {
    key: "dns_proto"
    value: "tcp"
  }
  latency_unit: "s"
  dns_probe {
    query_type: A
    resolved_domain: "google.com"
    resolve_first: true
    dns_proto: TCP
  }
}

[manugarg]

Thanks @aitorpazos! Looks mostly good, couple of minor suggestions.

[manugarg]

One last super minor cleanup, looks good otherwise :)

[manugarg]

Awesome. Thanks!