Enable brew service support for prometheus

formula extension method

```
def plist; <<~EOS
    <?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
    <plist version="1.0">
      <dict>
        <key>KeepAlive</key>
        <dict>
          <key>SuccessfulExit</key>
          <false/>
        </dict>
        <key>Label</key>
        <string>#{plist_name}</string>
        <key>ProgramArguments</key>
        <array>
          <string>#{opt_bin}/prometheus</string>
          <string>--config.file</string>
          <string>#{etc}/prometheus.yml</string>
        </array>
        <key>RunAtLoad</key>
        <true/>
        <key>WorkingDirectory</key>
        <string>#{var}</string>
        <key>StandardErrorPath</key>
        <string>#{var}/log/prometheus.log</string>
        <key>StandardOutPath</key>
        <string>#{var}/log/prometheus.log</string>
      </dict>
    </plist>
  EOS
end
```

homebrew.mxcl.prometheus.plist

```
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
  <dict>
    <key>KeepAlive</key>
    <dict>
      <key>SuccessfulExit</key>
      <false/>
    </dict>
    <key>Label</key>
    <string>homebrew.mxcl.prometheus</string>
    <key>ProgramArguments</key>
    <array>
      <string>/usr/local/opt/prometheus/bin/prometheus</string>
      <string>--config.file</string>
      <string>/usr/local/etc/prometheus.yml</string>
    </array>
    <key>RunAtLoad</key>
    <true/>
    <key>WorkingDirectory</key>
    <string>/usr/local/var</string>
    <key>StandardErrorPath</key>
    <string>/usr/local/var/log/prometheus.log</string>
    <key>StandardOutPath</key>
    <string>/usr/local/var/log/prometheus.log</string>
  </dict>
</plist>
```

prometheus.yml

```
global:
  scrape_interval:     15s # By default, scrape targets every 15 seconds.

  # Attach these labels to any time series or alerts when communicating with
  # external systems (federation, remote storage, Alertmanager).
  external_labels:
    monitor: 'codelab-monitor'

# A scrape configuration containing exactly one endpoint to scrape:
# Here it's Prometheus itself.
scrape_configs:
  # The job name is added as a label `job=<job_name>` to any timeseries scraped from this config.
  - job_name: 'prometheus'

    # Override the global default and scrape targets from this job every 5 seconds.
    scrape_interval: 5s

    static_configs:
      - targets: ['localhost:9090']
```

How to use ?

brew edit prometheus
copy  formula extension method to it
brew reinstall prometheus

```brew services start/restart prometheus``` will create  ~/Library/LaunchAgents/homebrew.mxcl.prometheus.plist, so don't need to copy it

touch /usr/local/etc/prometheus.yml
copy the content of prometheus.yml to it