Flask 400 decode error with HTTPHeader <nil> value

**Describe the bug**:
Flask transaction data are not gathered due to 400 decode error with HTTPHeader value
Dependency and metric data are gathered and shown on dashboard but transaction data are rejected.

Additional data just in case if it matters
- Flask debug is turned on but ELASTIC_APM_DEBUG is also set to True to enable sending data.

**Environment
- OS: Ubuntu Linux (Container to be exact)
- Python version: 3.10
- Framework and version [e.g. Django 2.1]: flask-3.0.2
- APM Server version: 8.15
- Agent version: 6.23.0

**Additional context**

Add any other context about the problem here.

- Agent config options

Click to expand

```
using default configs except for ELASTIC_APM_DEBUG
```

- `requirements.txt`:

Click to expand

```
Flask
SQLAlchemy
PyMySQL
Flask-Babel
pydantic
elastic-apm
```

Python Agent Logs
```
2024-10-04 01:50:43.785 7 DEBUG elasticapm.metrics [-] Collecting metrics base_metrics.py:97 collect
2024-10-04 01:50:44.176 7 DEBUG elasticapm.transport [-] flushing due to time since last flush 10.549s > max_flush_time 10.549s base.py:165 _process_queue
2024-10-04 01:50:44.183 7 DEBUG elasticapm.transport.http [-] Sent request, url=http://[apm-server-address]:8200/intake/v2/events size=25.74kb status=400 http.py:97 send
2024-10-04 01:50:44.184 7 ERROR elasticapm.transport [-] Failed to submit message: 'HTTP 400: {"accepted":32,"errors":[{"message":"decode error: data read error: v2.transactionRoot.Transaction: v2.transaction.Context: v2.context.Request: v2.contextRequest.URL: v2.contextRequestURL.Headers: invalid input for HTTPHeader: \\\\u003cnil\\\\u003e","document":"{\\\\"transaction\\\\": {\\\\"id\\\\": \\\\"956ba79c937359c5\\\\", \\\\"trace_id\\\\": \\\\"8930f92212c7229641b769754c1f13c5\\\\", \\\\"name\\\\": \\\\"GET [REDACTED]\\\\", \\\\"type\\\\": \\\\"request\\\\", \\\\"duration\\\\": 244.316, \\\\"result\\\\": \\\\"HTTP 2xx\\\\", \\\\"timestamp\\\\": 1728006633627826, \\\\"outcome\\\\": \\\\"success\\\\", \\\\"sampled\\\\": true, \\\\"span_count\\\\": {\\\\"started\\\\": 3, \\\\"dropped\\\\": 0}, \\\\"sample_rate\\\\": 1.0, \\\\"context\\\\": {\\\\"request\\\\": {\\\\"env\\\\": {\\\\"REMOTE_ADDR\\\\": \\\\"[REDACTED]\\\\", \\\\"
```

APM Server Logs
```
{"log.level":"error","@timestamp":"2024-10-04T01:51:25.015Z","log.logger":"request","log.origin":{"function":"github.com/elastic/apm-server/internal/beater/api.apmMiddleware.LogMiddleware.func1.1","file.name":"middleware/log_middleware.go","file.line":59},"message":"data validation error","service.name":"apm-server","url.original":"/intake/v2/events","http.request.method":"POST","user_agent.original":"apm-agent-python/6.23.0 (APP_NAME)","source.address":"[REDACTED]","http.request.id":"89ce0a55-95f1-4dd1-8b75-417a3bc5c990","event.duration":3209908,"http.request.body.bytes":20497,"http.response.status_code":400,"error.message":"decode error: data read error: v2.transactionRoot.Transaction: v2.transaction.Context: v2.context.Request: v2.contextRequest.URL: v2.contextRequestURL.Headers: invalid input for HTTPHeader: , decode error: data read error: v2.transactionRoot.Transaction: v2.transaction.Context: v2.context.Request: v2.contextRequest.URL: v2.contextRequestURL.Headers: invalid input for HTTPHeader: , decode error: data read error: v2.transactionRoot.Transaction: v2.transaction.Context: v2.context.Request: v2.contextRequest.URL: v2.contextRequestURL.Headers: invalid input for HTTPHeader: , decode error: data read error: v2.transactionRoot.Transaction: v2.transaction.Context: v2.context.Request: v2.contextRequest.URL: v2.contextRequestURL.Headers: invalid input for HTTPHeader: , decode error: data read error: v2.transactionRoot.Transaction: v2.transaction.Context: v2.context.Request: v2.contextRequest.URL: v2.contextRequestURL.Headers: invalid input for HTTPHeader: ","ecs.version":"1.6.0"}
```
Seems like agent is sending HTTPHeader as and apm server errors out when trying to decode the request.
I've redacted sensitive data tell me if you need more information.

Bắt đầu bằng cách tái hiện việc gửi transaction của Flask với Python 3.10, Flask 3.0.2, agent 6.23.0 và APM Server 8.15, sau đó kiểm tra cách các header của URL yêu cầu transaction được serialize. Hoàn thành khi dữ liệu transaction được intake endpoint chấp nhận thay vì trả về lỗi decode HTTPHeader .

Do mô hình lập chỉ mục viết ra từ nội dung của issue.