Changes

Misc_Helpers: Introduce API helper f_str_startswith

Counterpart of f_str_endswith(), this API is usually available in
several programming languages like python, and it's handy to have since
it's usually used and hence dev doesn't need to think on how to
implement it using regexp, etc.

This API will be used in a follow-up patch implementing Prometheus
metrics checker module.

Change-Id: Iad392ac652714a54a25954ea697f082d55772aa8