Use system_load15 metric for load-related alerts

Change-Id: I82b83d3c3badc7fa30b82310b8f6688d4cfa1972
Related-bug: PROD-34261
diff --git a/linux/meta/prometheus.yml b/linux/meta/prometheus.yml
index ba92ed0..865f744 100644
--- a/linux/meta/prometheus.yml
+++ b/linux/meta/prometheus.yml
@@ -40,7 +40,7 @@
       {%- endraw %}
       {%- set load_threshold = monitoring.system_load_threshold.warn|float %}
       if: >-
-        system_load5 / system_n_cpus > {{ load_threshold }}
+        system_load15 / system_n_cpus > {{ load_threshold }}
       {%- raw %}
       for: 5m
       labels:
@@ -53,11 +53,11 @@
       {%- endraw %}
       {%- set load_threshold = monitoring.system_load_threshold.crit|float %}
       if: >-
-        system_load5 / system_n_cpus > {{ load_threshold }}
+        system_load15 / system_n_cpus > {{ load_threshold }}
       {%- raw %}
       for: 5m
       labels:
-        severity: warning
+        severity: critical
         service: system
       annotations:
         summary: "System load is {%- endraw %}{{ load_threshold }}{%- raw %}"