Make query sampling possible by query

Original request by Peter Zaitsev:
"We have an option:

| log_slow_rate_limit | 1

Which allows to specify sampling so only portion of session is logged. It however is not helpful if you have just one session
such as replication thread or have app with few persistent connection.

Make an option something like log_slow_rate_limit_by which can be either "session" or "query" with session default to be same as now. Though I want to set it to "query" if I want to be sampling done on per query basics."

I propose add variable:
log_slow_rate_type="query|session"
for control this.