Alerting rules docs reference $labels, but only .Labels appear to work

[mkadin]

Bug Report

What did you do?
I set up a rule that looks like this:

  - alert: Server memory utilization
    expr: (node_memory_MemTotal{instance=~"(job|app|web).*"} -node_memory_MemAvailable{instance=~"(job|app|web).*"})/ node_memory_MemTotal{instance=~"(job|app|web).*"} * 100 > 80
    for: 5m
    labels:
      service: server
      severity: high
    annotations:
      summary: {{$labels.instance}} memory usage > 80%

What did you expect to see?
For this configuration to be valid, based on the docs at https://github.com/prometheus/prometheus/blob/master/docs/configuration/alerting_rules.md

What did you see instead? Under which circumstances?
Prometheus crashed on startup:

level=info ts=2018-11-12T18:02:26.966132819Z caller=targetmanager.go:71 component="target manager" msg="Starting target manager..."
level=info ts=2018-11-12T18:02:27.553405224Z caller=main.go:326 msg="TSDB started"
level=info ts=2018-11-12T18:02:27.553995989Z caller=main.go:394 msg="Loading configuration file" filename=/etc/prometheus/prometheus.yml
level=error ts=2018-11-12T18:02:27.557580751Z caller=manager.go:485 component="rule manager" msg="loading groups failed" err="yaml: line 10: did not find expected key"
level=error ts=2018-11-12T18:02:27.557859605Z caller=main.go:413 msg="Failed to apply configuration" err="error loading rules, previous rule set restored"
level=error ts=2018-11-12T18:02:27.558245459Z caller=main.go:356 msg="Error loading config" err="one or more errors occurred while applying the new configuration (--config.file=/etc/prometheus/prometheus.yml)"

Environment
ubuntu 16.04

• System information:

ubuntu@infra1-us-east-2:~$ uname -srm
Linux 4.4.0-1038-aws x86_64

• Prometheus version:

ubuntu@infra1-us-east-2:~$ /src/prometheus/prometheus-server/prometheus --version
prometheus, version 2.0.0 (branch: HEAD, revision: 0a74f98628a0463dddc90528220c94de5032d1a0)
  build user:       root@615b82cb36b6
  build date:       20171108-07:11:59
  go version:       go1.9.2

• Alertmanager version:

ubuntu@infra1-us-east-2:~$ /src/prometheus/prometheus-server/prometheus --version
prometheus, version 2.0.0 (branch: HEAD, revision: 0a74f98628a0463dddc90528220c94de5032d1a0)
  build user:       root@615b82cb36b6
  build date:       20171108-07:11:59
  go version:       go1.9.2

• Prometheus configuration file:
  see above

• Alertmanager configuration file:
  see above

[mkadin]

I should have added, once I changes $labels to .Labels, it worked properly! Did this variable change names and the docs need updating?

[brian-brazil]

That's a YAML issue, try quoting the template string.

[mkadin]

You're right...sorry for the bad report!