Return-value matching

[gomoripeti]

Allow to filter functions based on their return value (instead of arguments).

Apart from the XProf-flavoured match-spec takes an additional parameter
RetMatchFun which matches the value returned by the function. The call latency
is only measured if there is a match.

RetMatchFun receives the value returned by the traced function as a single
argument and can return:

• false: no match, don't measure
• true: match, measure and capture the original return value.
• {true, NewValue}: match, measure and capture NewValue instead of the original return value.

If RetMatchFun is arity-2 it could match on an exception (class and reason). Should an arity-1 RetMatchFun match on exceptions of form {Class, Reason} too?
We can think of some shortcuts eg. as a convenience function_clause could mean no match (don't have to handle all return values, only the positive case, as in the second example below)

Query example (suggestion):

#retmatch matchfun: fun(List) -> lists:member(12, List) end, mfa: ...
#retmatch matchfun: fun(error, badarg) -> true end, mfa: ...

It would be useful if the graph would show both a total count (number of
return_trace messages) and matched count (only when matchfun matches) to see a
rate how oftern thr return value matches.