[counters] Add support for string values

Add new function mono_counters_register_with_size which let the counters API now what is going to be the max size of the string.
The type of the value passed when MONO_COUNTER_STRING is passed must be char* (instead of char** previously). This is to guarantee that, if we want to change the value of the counter, we do not reallocate and free the old value, but we change the value in place. Thus to avoir race condition and access to freed memory.
[counters] Fix potential overflow in string sampling
Merge pull request #1022 from ludovic-henry/pr9-string-counters

[counters] Add support for string values