Prometheus rules

Rules can be used to create new metrics, or to trigger alerts.

Documentation

official docs

https://prometheus.io/docs/prometheus/latest/configuration/recording_rules/

Custom Metric

# /usr/local/etc/memory_rules.yml

groups:
  - name: available_memory
    rules:
    - record: node_available_memory_percent                    # metric rule-result is exposed as
      expr:   node_memory_free_bytes / node_memory_size_bytes  # PromQL query rule performs

# /usr/local/etc/prometheus.yml

rule_files:
  - "memory_rules.yml"

service prometheus restart

now node_available_memory_percent metric should be cached and queryable.