请问一下，在 Prometheus 监控系统中，告警规则： expr: job:EC2 - Node:up == 0 表示什么意思呢？

[huoshuaibing]

groups:
- name: host-alerts
rules:
- alert: InstanceDown
expr: job:EC2 - Node:up == 0
for: 1m
labels:
severity: critical
annotations:
text: |
{{ $labels.instance }} of job {{ $labels.job }} has been down for more than 1 minute.

[CivAx]

当 EC2 节点持续 1 分钟检测到节点（ node ）在线数量为 0 时，以 Critical 级别告警并发送 text 告警信息