Text format execution plan on dashboard misses dynamic partition info

## Enhancement
```
mysql> create table t(a int, b int) partition by range(a) (
-> partition p0 values less than (10),
-> partition p1 values less than (20),
-> partition p2 values less than (30),
-> partition p3 values less than maxvalue
-> );
Query OK, 0 rows affected (0.12 sec)

mysql> set @@tidb_partition_prune_mode = 'dynamic';
Query OK, 0 rows affected (0.00 sec)

mysql> explain select max(a) from t where a > 25;
+--------------------------------+----------+-----------+-----------------+-----------------------------------------+
| id | estRows | task | access object | operator info |
+--------------------------------+----------+-----------+-----------------+-----------------------------------------+
| StreamAgg_11 | 1.00 | root | | funcs:max(test.t.a)->Column#4 |
| └─TopN_12 | 1.00 | root | | test.t.a:desc, offset:0, count:1 |
| └─TableReader_20 | 1.00 | root | partition:p2,p3 | data:TopN_19 |
| └─TopN_19 | 1.00 | cop[tikv] | | test.t.a:desc, offset:0, count:1 |
| └─Selection_18 | 3333.33 | cop[tikv] | | gt(test.t.a, 25), not(isnull(test.t.a)) |
| └─TableFullScan_17 | 10000.00 | cop[tikv] | table:t | keep order:false, stats:pseudo |
+--------------------------------+----------+-----------+-----------------+-----------------------------------------+
6 rows in set (0.00 sec)
```
You can see that `TableReader_20` shows the dynamic partition info `partition:p2,p3` in `access object` field.

However, when you see the execution plan in text format on dashboard, the dynamic partition info `partition:p2,p3` is missing.