MDEV-34190: r_engine_stats.pages_read_count is unrealistically low

The symptoms were: take a server with no activity and a table that's
not in the buffer pool. Run a query that reads the whole table and
observe that r_engine_stats.pages_read_count shows about 2% of the table
was read. Who reads the rest?

The cause was that page prefetching done inside InnoDB was not counted.

This counts page prefetch requests made in buf_read_ahead_random() and
buf_read_ahead_linear() and makes them visible in:

- ANALYZE: r_engine_stats.pages_prefetch_read_count
- Slow Query Log: Pages_prefetched:

This patch intentionally doesn't attempt to count the time to read the
prefetched pages:
* there's no obvious place where one can do it
* prefetch reads may be done in parallel (right?), it is not clear how
  to count the time in this case.

mysql-test/include/log_slow_grep.inc

@@ -22,7 +22,7 @@
 
 # InnoDB/Engines
 --let log_expected_matches = $log_slow_verbosity_innodb_expected_matches
---let grep_pattern = ^# Pages_accessed: \d+  Pages_read: \d+  Pages_updated: \d+  Old_rows_read: \d+\$
+--let grep_pattern = ^# Pages_accessed: \d+  Pages_read: \d+  Pages_prefetched: \d+  Pages_updated: \d+  Old_rows_read: \d+\$
 --source include/log_grep.inc
 
 --let grep_pattern = ^# Pages_read_time: \d+\.\d+  Engine_time: \d+\.\d+\$