Add examples of using conditional logic in messageFn params

[sanderson]

A community user is trying to conditionally set a message based off values in the row. This is for the messageFn parameter of monitor.check(). We need to show the syntax for that conditional logic:

(r) => {
  message = (
    if r._level == "crit" then "Critical alert!! Disk usage for ${r.path} on ${r.host} is at ${string(v: r._value)}%!"
    else "Things are looking good."
  )
  return message
}

Relevant URLs

• https://docs.influxdata.com/influxdb/v2.0/reference/flux/stdlib/monitor/check/
• https://docs.influxdata.com/influxdb/cloud/reference/flux/stdlib/monitor/check/
• https://docs.influxdata.com/influxdb/v2.0/monitor-alert/custom-checks/
• https://docs.influxdata.com/influxdb/cloud/monitor-alert/custom-checks/