services: Set Nomad's User-Agent by default on HTTP checks for nomad services

[shoenig]

This PR sets the default User-Agent on checks of nomad services to the common UA for Nomad stuff (e.g. "Nomad/1.5.0-dev (+https://www.nomadproject.io/; go1.20)"). If the user supplies a UA that one takes precedence.

[shoenig]

Spot check

job without setting User-Agent header

job "ua" {
  group "group" {

    network {
      mode = "bridge"
      port "http" { to = 9090 }
    }

    service {
      name     = "fakeservice"
      port     = "http"
      provider = "nomad"
      check {
        name = "check"
        type     = "http"
        path     = "/"
        interval = "5s"
        timeout  = "1s"
      }
    }

    task "nj" {
      driver = "docker"

      config {
        image = "nicholasjackson/fake-service:v0.23.1"
        ports = ["http"]
      }

      env {
        LISTEN_ADDR = "0.0.0.0:9090"
        LOG_FORMAT  = "text"
      }

      resources {
        cpu    = 10
        memory = 32
      }
    }
  }
}

logs

2023-02-23T13:10:09.747Z [INFO]  Handle inbound request: request="GET / HTTP/1.1
Host: 127.0.0.1:29784
user-agent: Nomad/1.5.0-dev (+https://www.nomadproject.io/; go1.20)
accept-encoding: gzip"
2023-02-23T13:10:09.747Z [INFO]  Finished handling request: duration=71.256µs

now with User-Agent header set

check {
  name = "check"
  type     = "http"
  path     = "/"
  interval = "5s"
  timeout  = "1s"
  header {
    User-Agent = ["my-custom-agent"]
  }
}

logs

2023-02-23T13:11:45.642Z [INFO]  Handle inbound request: request="GET / HTTP/1.1
Host: 127.0.0.1:23390
user-agent: my-custom-agent
accept-encoding: gzip"
2023-02-23T13:11:45.642Z [INFO]  Finished handling request: duration=70.718µs

[tgross]

LGTM!

[tgross]

Augh, the lack of common headers in net/http constants!