Prevent metric from being computed before updated using __new__

[trsvchn]

Addresses #336 #2007

Description:

Prevents metric from being computed before updated using __new__ magic.

__new__ is called by default on instance creation before __init__ call. Thus, we wrap methods before instance instantiation.

Check list:

• New tests are added (if a new feature is added)
• New doc strings: description and/or example code are in RST format
• Documentation is updated (if required)