[snappi]: Fix for rx_l1_rate_gbps index error issue in traffic_generation file. (sonic-net#72)

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### Description of PR
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Summary:
The metric 'Rx_L1_rate_Gbps' is picked from traffic item statistics. However, in 10.80, this is not working causing index error.

Switching it to Rx_L1_rate_bps in the statistics file.

Fixes # (issue)
sonic-net#16731

### Type of change

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- [x] Bug fix
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- [X] Bug fix
- [ ] Testbed and Framework(new/improvement)
- [ ] New Test case
 - [ ] Skipped for non-supported platforms
- [ ] Test case improvement

### Back port request
- [ ] 202012
- [ ] 202205
- [ ] 202305
- [ ] 202311
- [X] 202405
- [X] 202411

### Approach
#### What is the motivation for this PR?
Resolving index error in the traffic item statistics. Instead of capturing Gbps rates, which no longer exists, switched to bps rates.

#### How did you do it?
Single line of code:
```
stats[metric_name+'_Rx_L1_Rate_bps'] = float(metric['Rx L1 Rate (bps)'])
```

#### How did you verify/test it?
Local clone.

#### Any platform specific information?

#### Supported testbed topology if it's a new test case?

### Documentation
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Author: mssonicbld

tests/common/snappi_tests/traffic_generation.py

@@ -912,7 +912,7 @@ def tgen_curr_stats(traf_metrics, flow_metrics, data_flow_names):
         stats[metric_name+'_txrate_Gbps'] = tx_rate_gbps
         stats[metric_name+'_rxrate_fps'] = float(metric['Rx Frame Rate'])
         stats[metric_name+'_rxrate_Gbps'] = rx_rate_gbps
-        stats[metric_name+'_LI_rxrate_Gbps'] = float(metric['Rx L1 Rate (Gbps)'])
+        stats[metric_name+'_Rx_L1_Rate_bps'] = float(metric['Rx L1 Rate (bps)'])
 
     for metric in flow_metrics:
         if metric.name not in data_flow_names: