Summary:
After commit 5b193c8d587f887f3791f76e3ff4e3112a023bb1 `CatalogManager::RegisterNewTabletsForSplit` started logging empty partition keys for child tablets:
```
I0312 09:50:35.742720 67219 catalog_manager.cc:7700] Registered new tablet 270f1df4dce5420aa8470c8d3476f2b9 (partition_key_start: , partition_key_end: , split_depth: 1) to split the tablet 2e8282a579ab49248b2628a77b61a950 (partition_key_start: "" partition_key_end: "") for table ql_client_test_table [id=4e77c5700b3545f6b8ac1c2194a129c3], new partition_list_version: 1
I0312 09:50:35.742772 67219 catalog_manager.cc:7700] Registered new tablet f89f93f1fb254e49a9a2d9286c6e60ed (partition_key_start: , partition_key_end: , split_depth: 1) to split the tablet 2e8282a579ab49248b2628a77b61a950 (partition_key_start: "" partition_key_end: "") for table ql_client_test_table [id=4e77c5700b3545f6b8ac1c2194a129c3], new partition_list_version: 1
I0312 09:50:36.262280 67219 catalog_manager.cc:7700] Registered new tablet 2c32902243284083a1d48e229ec19ebc (partition_key_start: , partition_key_end: , split_depth: 2) to split the tablet f89f93f1fb254e49a9a2d9286c6e60ed (partition_key_start: "\220\244" partition_key_end: "") for table ql_client_test_table [id=4e77c5700b3545f6b8ac1c2194a129c3], new partition_list_version: 2
I0312 09:50:36.262315 67219 catalog_manager.cc:7700] Registered new tablet d63c5f1ee23a4f4eb2a4790d7816d145 (partition_key_start: , partition_key_end: , split_depth: 2) to split the tablet f89f93f1fb254e49a9a2d9286c6e60ed (partition_key_start: "\220\244" partition_key_end: "") for table ql_client_test_table [id=4e77c5700b3545f6b8ac1c2194a129c3], new partition_list_version: 2
```
This was due to logging was moved before `new_tablet->mutable_metadata()->CommitMutation()` where partition info is not yet committed. This issue only affects logging but not correctness.
Fixed this by moving logging after mutation commit and also improved it to show hex strings:
```
I0317 15:26:12.795843 1059935 catalog_manager.cc:7730] Registered new tablet ebb46a4b831941889973c0694a4ac8cf (partition_key_start: , partition_key_end: 90A4, split_depth: 1) to split the tablet fddb3a4ff4074c1d867754af038085ed (partition_key_start: , partition_key_end: ) for table ql_client_test_table [id=7b9161f8592f425c8b4ab98ef2f1a449], new partition_list_version: 1
I0317 15:26:12.795887 1059935 catalog_manager.cc:7730] Registered new tablet ff19615fb4e54c20a49e1316ef5f3c5b (partition_key_start: 90A4, partition_key_end: , split_depth: 1) to split the tablet fddb3a4ff4074c1d867754af038085ed (partition_key_start: , partition_key_end: ) for table ql_client_test_table [id=7b9161f8592f425c8b4ab98ef2f1a449], new partition_list_version: 1
I0317 15:26:13.655956 1059935 catalog_manager.cc:7730] Registered new tablet 5bdec4b37d2d43f1b514e5f33cdc09cb (partition_key_start: 90A4, partition_key_end: CC68, split_depth: 2) to split the tablet ff19615fb4e54c20a49e1316ef5f3c5b (partition_key_start: 90A4, partition_key_end: ) for table ql_client_test_table [id=7b9161f8592f425c8b4ab98ef2f1a449], new partition_list_version: 2
I0317 15:26:13.655985 1059935 catalog_manager.cc:7730] Registered new tablet d1b42945d01f458fa736862c0d79eb43 (partition_key_start: CC68, partition_key_end: , split_depth: 2) to split the tablet ff19615fb4e54c20a49e1316ef5f3c5b (partition_key_start: 90A4, partition_key_end: ) for table ql_client_test_table [id=7b9161f8592f425c8b4ab98ef2f1a449], new partition_list_version: 2
```
Jira: DB-15771
Test Plan: TabletSplitITest.SplitSingleTabletWithLimit
Reviewers: asrivastava
Reviewed By: asrivastava
Subscribers: ybase
Tags: #jenkins-ready
Differential Revision: https://phorge.dev.yugabyte.com/D42580
