Observability is the ability to:

• Which services did the request go
• What did every service do when processing the request
• If the request was slow, where were the bottlenecks
• If the request failed, where did the errors happen
• How different was the execution from the normal system behavior (structural vs performance)
• What was on the critical path of the request
• Who should be paged

Source: here. For Distributed Tracing, use Jaeger.