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.

sql/sql_explain.cc

@@ -1727,6 +1727,8 @@ static void trace_engine_stats(handler *file, Json_writer *writer)
     if (hs->pages_read_time)
       writer->add_member("pages_read_time_ms").
         add_double(hs->pages_read_time * 1000. / timer_tracker_frequency());
+    if (hs->pages_prefetched)
+      writer->add_member("pages_prefetch_read_count").add_ull(hs->pages_prefetched);
     if (hs->undo_records_read)
       writer->add_member("old_rows_read").add_ull(hs->undo_records_read);
     writer->end_object();