Adjust OOM kill detected rule (#495)

* Adjust OOM kill detected rule

When a machine runs out of memory, it happens that the node
exporter stops responding for multiple minutes. I've adjusted
the rule now to take this into account: even if it takes 15-20
minutes before the machine becomes responsive again, the
alert should still fire.

* Update rules.yml

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Co-authored-by: Samuel Berthe <dev@samuel-berthe.fr>

Author: perlun

_data/rules.yml

@@ -271,8 +271,10 @@ groups:
                 severity: info
               - name: Host OOM kill detected
                 description: OOM kill detected
-                query: "(increase(node_vmstat_oom_kill[1m]) > 0)"
+                query: "(increase(node_vmstat_oom_kill[30m]) > 0)"
                 severity: warning
+                comments: |
+                  When a machine runs out of memory, the node exporter can become unresponsive for several minutes. Even if the system takes 15–20 minutes to recover, the alert should still trigger.
               - name: Host EDAC Correctable Errors detected
                 description: 'Host {{ $labels.instance }} has had {{ printf "%.0f" $value }} correctable memory errors reported by EDAC in the last 5 minutes.'
                 query: "(increase(node_edac_correctable_errors_total[1m]) > 0)"