ISSUE-724: Add PhysicalAddr() method to interface Producer and Consumer for client-side metric report?

[sijie]

Original Issue: apache#724

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Is your feature request related to a problem? Please describe.
No

Describe the solution you'd like
Add method PhysicalAddr() to interface Producer and interface Consumer which exposes the broker address to which the producer or consumer connect to.

From the client view, we need to report some metrics such as time-cost, return code and so on everytime we send or consume a msg. It's neccesary to distinguish such metrics among difference brokers for a better observation. While there is no way for the user to obtain the address of the broker connected to, now.

So I think it's useful to add PhysicalAddr() to Producer and Consumer interface.