[proflog] Add counters to proflog type enumeration
[proflog] Add counters options parsing to proflog
[proflog] Add emit_string and emit_double utility functions
[proflog] Add counters functions

Two main functions :
 - counters_init : initalize the "counters" variable with all the counters registered durint the runtime initialization, called after runtime initialization
 - counters_sample : samples the counters values, called every "counters_interval" ms
[proflog] Add counters sampling to helper thread loop
[proflog] Fix double dumping and compilation warning
[proflog] Fix time issue on sampling
[counters] Add synthetic counters

After discussion with Kumpera, it was decided to load them lazily when calling mono_counters_foreach for the first time. It has to be inside the mono runtime, and not in a profiler because it accesses proclib API which is all MONO_INTERNAL functions.