feat(lib/policy/expressions): add system load average to bot expression inputs (#766)

* feat(lib/policy/expressions): add system load average to bot expression inputs

This lets Anubis dynamically react to system load in order to
increase and decrease the required level of scrutiny. High load? More
scrutiny required. Low load? Less scrutiny required.

* docs: spell system correctly

Signed-off-by: Xe Iaso <me@xeiaso.net>

* Update metadata

check-spelling run (pull_request) for Xe/load-average

Signed-off-by: check-spelling-bot <check-spelling-bot@users.noreply.github.com>
on-behalf-of: @check-spelling <check-spelling-bot@check-spelling.dev>

* fix(default-config): don't enable low load average feature by default

Signed-off-by: Xe Iaso <me@xeiaso.net>

---------

Signed-off-by: Xe Iaso <me@xeiaso.net>
Signed-off-by: check-spelling-bot <check-spelling-bot@users.noreply.github.com>
Signed-off-by: Xe Iaso <xe.iaso@techaro.lol>

data/botPolicies.yaml

@@ -74,6 +74,25 @@ bots:
     weight:
       adjust: 10
 
+  ## System load based checks.
+  # If the system is under high load, add weight.
+  - name: high-load-average
+    action: WEIGH
+    expression: load_1m >= 10.0 # make sure to end the load comparison in a .0
+    weight:
+      adjust: 20
+
+  ## If your backend service is running on the same operating system as Anubis,
+  ## you can uncomment this rule to make the challenge easier when the system is
+  ## under low load.
+  ##
+  ## If it is not, remove weight.
+  # - name: low-load-average
+  #   action: WEIGH
+  #   expression: load_15m <= 4.0 # make sure to end the load comparison in a .0
+  #   weight:
+  #     adjust: -10
+
   # Generic catchall rule
   - name: generic-browser
     user_agent_regex: >-