title: Cannot start prometheus by using systemd
body: <p>OS level: <code>CentOS Linux release 7.4.1708</code><br/>
  Prometheus level: <code>2.4.2</code><br/>
  prometheus.service:</p>
  
  <pre><code>[Unit]
  Description=Prometheus
  
  [Service]
  User=prometheus
  ExecStart=/usr/local/prometheus/prometheus
  
  [Install]
  WantedBy=default.target
  </code></pre>
  
  <p>When I use <code>systemctl start prometheus</code> to start the prometheus service, it always exit the main process by itself. And the systemctl's log shows like this:</p>
  
  <pre><code>● prometheus.service - Prometheus
     Loaded: loaded (/etc/systemd/system/prometheus.service; disabled; vendor preset: disabled)
     Active: failed (Result: exit-code) since Tue 2018-09-25 10:43:56 CST; 6s ago
    Process: 5174 ExecStart=/usr/local/prometheus/prometheus (code=exited, status=1/FAILURE)
   Main PID: 5174 (code=exited, status=1/FAILURE)
  
  Sep 25 10:43:56 devtestserver systemd[1]: Started Prometheus.
  Sep 25 10:43:56 devtestserver systemd[1]: Starting Prometheus...
  Sep 25 10:43:56 devtestserver prometheus[5174]: level=info ts=2018-09-25T02:43:56.736457704Z caller=main.go:238 msg="Starting Prometheus" version="(version=2.4.2, branch=HE...13b1190a0)"
  Sep 25 10:43:56 devtestserver systemd[1]: prometheus.service: main process exited, code=exited, status=1/FAILURE
  Sep 25 10:43:56 devtestserver systemd[1]: Unit prometheus.service entered failed state.
  Sep 25 10:43:56 devtestserver systemd[1]: prometheus.service failed.
  Hint: Some lines were ellipsized, use -l to show in full.
  </code></pre>
  
  <p>I have no ideas with this problem. I use the same config for the <code>node_exporter</code>,  but <code>node_exporter</code> can start as normal. Please help. Thanks a lot.</p>