annotation to store timestamp,label pair

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on March 01, 2011 11:11:37_

for the problem of determining which part of program execution (e.g.,
subtest for a chromium unit test executable) allocated memory that was
later leaked, it's expensive to ask for a leak scan after every subtest.
one solution would be to store a timestamp w/ each allocation (issue #173: does
have a memory usage cost) and add an annotation to store a timestampe,label
pair that can be called after each subtest.

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=311_