[proflog] Add emit_string and emit_double functions
[proflog] Add counters option parsing
[proflog] Add counters enum type and documentation
[proflog] Add counters_init and counters_sample functions
[proflog] Add integration of counters_init and counters_sample

counters_init is called at the end of runtime initialization, on runtime_initialized hook
counters_sample is called in the helper_thread at least once per second
[proflog] Increment LOG_DATA_VERSION
[proflog] Edit according to Paolo review
Merge pull request #1024 from ludovic-henry/pr10-proflog-counters

[proflog] Add counters support to proflog