[Proposal] Design of tracing example and tasks.

[LaurenceLiZhixin]

1. We should add an example 'tracing' to show the feature of 'whole calling stacks' tracing by open-tracing protocol.
2. We should add an task to docs, to show how to troubleshoot time-cost problems based on a real-scene example of E-commerce scenario.
3. We can explore a tracing capability based on iocli trace command, without using third-party collection server such as Jaeger, and support full-link real-time tracing capability through RPC protocol and Debug AOP layer.