MDEV-8840: ANALYZE FORMAT=JSON produces wrong data with BKA

The issue was that r_loops, r_rows and r_filtered in ANALYZE FORMAT= JSON were not
calculated for the table on which we were performing the MRR scan in the BKA join
Fixed this by adding respective counter in the JOIN_TAB_SCAN_MRR::open and JOIN_TAB_SCAN_MRR::next

mysql-test/r/explain_json.result

@@ -1524,12 +1524,12 @@ ANALYZE
         "key_length": "5",
         "used_key_parts": ["a"],
         "ref": ["test.t3.a"],
-        "r_loops": 0,
+        "r_loops": 1,
         "rows": 1,
-        "r_rows": null,
+        "r_rows": 10,
         "r_total_time_ms": "REPLACED",
         "filtered": 100,
-        "r_filtered": null,
+        "r_filtered": 100,
         "index_condition_bka": "((t4.b + 1) <= (t3.b + 1))"
       },
       "buffer_type": "flat",

sql/sql_join_cache.cc

@@ -3877,6 +3877,7 @@ int JOIN_TAB_SCAN_MRR::open()
   /* Dynamic range access is never used with BKA */
   DBUG_ASSERT(join_tab->use_quick != 2);
 
+  join_tab->tracker->r_scans++;
   save_or_restore_used_tabs(join_tab, FALSE);
 
   init_mrr_buff();
@@ -3920,6 +3921,8 @@ int JOIN_TAB_SCAN_MRR::next()
   int rc= join_tab->table->file->multi_range_read_next((range_id_t*)ptr) ? -1 : 0;
   if (!rc)
   {
+    join_tab->tracker->r_rows++;
+    join_tab->tracker->r_rows_after_where++;
     /*
       If a record in in an incremental cache contains no fields then the
       association for the last record in cache will be equal to cache->end_pos