feat: avoid confusion in desc table

[fengjiachun]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

In the output of DESC TABLE, we have used the name Field (following MySQL's convention), but our Time Series Data Model also includes Field (Tag, Field, Timestamp). This may confuse users.

before

+-----------+----------------------+-----+------+---------+---------------+
| Field     | Type                 | Key | Null | Default | Semantic Type |
+-----------+----------------------+-----+------+---------+---------------+
| ts        | TimestampMillisecond | PRI | NO   |         | TIMESTAMP     |
| collector | String               | PRI | YES  |         | TAG           |
| host      | String               | PRI | YES  |         | TAG           |
| val       | Float64              |     | YES  |         | FIELD         |
+-----------+----------------------+-----+------+---------+---------------+

after

+-----------+----------------------+-----+------+---------+---------------+
| Column    | Type                 | Key | Null | Default | Semantic Type |
+-----------+----------------------+-----+------+---------+---------------+
| ts        | TimestampMillisecond | PRI | NO   |         | TIMESTAMP     |
| collector | String               | PRI | YES  |         | TAG           |
| host      | String               | PRI | YES  |         | TAG           |
| val       | Float64              |     | YES  |         | FIELD         |
+-----------+----------------------+-----+------+---------+---------------+

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 #2272 (529ea69) into develop (96fd17a) will decrease coverage by 0.42%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #2272      +/-   ##
===========================================
- Coverage    85.16%   84.74%   -0.42%     
===========================================
  Files          709      709              
  Lines       116457   116457              
===========================================
- Hits         99179    98691     -488     
- Misses       17278    17766     +488