The discrete feature values in the gcformat sample data generated by the OpenMLDB SQL feature extraction script are inconsistent with those calculated by the PICO script

**Bug Description**
Service Version: 0.9.0
The discrete feature values in the gcformat sample data generated by the OpenMLDB SQL feature extraction script are inconsistent with those calculated by the PICO script.

**Expected Behavior**
Current incorrect format: label| slot:sign:origin-value
Correct format: label index| slot:sign:origin-value

**Relation Case**
OpenMLDB SQL Feature Extraction Example:
```
0| 1:0:1 2:4599670039981440374 3:6365000770384461703 4:0:93.200000
1| 1:0:2 2:5613161932270271752 3:-1384602352766124944 4:0:93.075000
0| 1:0:3 2:4599670039981440374 3:-6239076729344379818 4:0:92.893000
```
PICO Feature Extraction Example:
```
0 0| 2:-8773247204422130117:1 3:4042412524814531440 4:6048373541161169225 5:4681710344575317709:0x1.74ccccccccccdp6
1 1| 2:-8773247204422130117:2 3:6142047291687075953 4:1461111459061395210 5:4681710344575317709:0x1.744cccccccccdp6
0 2| 2:-8773247204422130117:3 3:4042412524814531440 4:3353218529862650678 5:4681710344575317709:0x1.73926e978d4fep6
```

**Steps to Reproduce**
1. data schema:
```
id[Int],age[Int],job[String],cons_price_idx[Double],y[Int]
```
2. PICO Feature Extraction Script：
```
target_y = binary_label(y)
f_id = continuous(id)
f_age = discrete(age)
f_job = discrete(job)
f_cons_price_idx = continuous(cons_price_idx)
```
4. OpenMLDB SQL Feature Extraction Script：
```
select gcformat(
binary_label(bool(y)),
continuous(id),
discrete(age),
discrete(job),
continuous(cons_price_idx)
) as instance from main_table
```

The issue is about the gcformat output in OpenMLDB SQL. Examine the SQL feature extraction script and the underlying C++ implementation of the gcformat function. Compare the output format with the PICO script's expected format. The fix likely involves modifying the string formatting logic for discrete features to include the label index. Look for tests related to gcformat or feature extraction to validate the change.

Rédigé par le modèle d'indexation à partir du texte de l'issue.