fix log stack overflow if metadata contains circular reference

[wangzlei]

Issue #, if available:
When segment metadata contains objects having circular reference, the segment serialization will get RecursionError exception. Then the log.exception in conversion.py will print infinite logs util hitting a new RecursionError exception, causes log flood in user application logs.

Description of changes:
Using pprint instead of normal logging. pprint can handle cycles because it tracks objects it has already visited and, when it detects a recursive reference, it prints a placeholder like <Recursion on ...> instead of recursing, preventing log flood. The output looks like

Caught exception: dict contains cycle attribute: payload -> {'name': 'loop', 'self': <Recursion on dict with id=4414423360>} attribute: _recorded -> True attribute: _cause_id -> '6dd1416f68a1c043' gc.collect() returned: 51732 ---------------------------------------- Caught exception: dict contains cycle attribute: payload -> {'name': 'loop', 'self': <Recursion on dict with id=4420684992>} attribute: _recorded -> True attribute: _cause_id -> 'ac93f095f48beedf' gc.collect() returned: 6 

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[vastin]

LGTM