fix: generate region path with given prefix

[waynexia]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

#2399 misuses the region storage path prefix. After this fix the server can re-open previous created regions:

MySQL [(none)]> show tables;
+---------+
| Tables  |
+---------+
| numbers |
| scripts |
+---------+
2 rows in set (0.013 sec)

MySQL [(none)]> create table t(ts timestamp time index);
Query OK, 0 rows affected (0.005 sec)

MySQL [(none)]> show tables;
+---------+
| Tables  |
+---------+
| numbers |
| scripts |
| t       |
+---------+
3 rows in set (0.001 sec)

MySQL [(none)]> show tables;
ERROR 2013 (HY000): Lost connection to server during query
MySQL [(none)]> show tables;
ERROR 2006 (HY000): Server has gone away
No connection. Trying to reconnect...
Connection id:    8
Current database: *** NONE ***

+---------+
| Tables  |
+---------+
| numbers |
| scripts |
| t       |
+---------+
3 rows in set (0.015 sec)

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.

Refer to a related PR or issue link (optional)

[codecov]

Codecov Report

Merging #2409 (b817ee4) into develop (c149c12) will decrease coverage by 0.07%.
The diff coverage is 0.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2409      +/-   ##
===========================================
- Coverage    77.57%   77.51%   -0.07%     
===========================================
  Files          718      718              
  Lines       113622   113623       +1     
===========================================
- Hits         88143    88075      -68     
- Misses       25479    25548      +69     

[killme2008]

The key point is that we should create a test to cover this scenario, it's very important.

[killme2008]

Let's merge this PR first, please add a test to cover this issue #2407

[v0y4g3r]

LGTM