Add observability doc

README.md

@@ -43,6 +43,4 @@ Useful links for programming
     - tree
     - ...
 
-### Observability
+### [Observability](observability.md)
-
-[see](observability.md)

observability.md

@@ -1 +1,37 @@
 # Observability
+
+## Logs
+
+- Unify code to log - configure to include module name: https://hexdocs.pm/logger/Logger.html
+- Failures
+- Audit logs
+
+## Metrics
+
+- Take time to calculate - they aren't free
+- Measure time
+- If something happens: do the metrics reflect the state you envisioned?
+    - Yes: good
+    - No: bad metric
+
+
+- Types of metrics
+    - Business
+        - Sales
+        - Marketing
+    - System
+        - CPU
+        - RAM
+        - Process count
+        - Performance
+        - Availability
+        - ...
+    - Product
+
+- Metrics kinds
+    - Leading (short time): test hypothesis 
+    - Lagging (long time): whether actions were successful
+
+## Alarms
+
+https://www.erlang.org/doc/apps/sasl/alarm_handler.html