ECS message field will be populated with the OTEL exception.message field instead of the body field

[HHobeck]

Describe the bug
I'm using a OTEL collector to forward the logging messages to elastic (via the APM module) and see that the message field is not filled correctly when providing exception details.

To Reproduce
Execute a dotnet core 6.0 application with the following C# code and send it with OpenTelemetry.Exporter.OpenTelemetryProtocol to the elastic APM server:

var logger = loggerFactory.CreateLogger(GetType());
try
{
    throw new InvalidOperationException();
}
catch (Exception exception)
{
    logger.LogError(exception, "Hello {World}", "Universe");
    throw;
}

I'm using elastic in version:
v 8.13.4

Actual behavior
The message of the log entry in elastic is of value: Operation is not valid due to the current state of the object.

Expected behavior
I would expect the value: Hello Universe

Debug logs
The resulting OTEL message send to the APM server is:

{
    "resourceLogs": [
        {
            "resource": {
                "attributes": [
                    {
                        "key": "deployment.environment",
                        "value": {
                            "stringValue": "Staging"
                        }
                    },
                    {
                        "key": "service.name",
                        "value": {
                            "stringValue": "LoggingExample"
                        }
                    },
                    {
                        "key": "service.namespace",
                        "value": {
                            "stringValue": "LoggingExample"
                        }
                    },
                    {
                        "key": "service.instance.id",
                        "value": {
                            "stringValue": "19d619b4-a623-45a4-97bb-62e59a14d7ef"
                        }
                    },
                    {
                        "key": "telemetry.sdk.name",
                        "value": {
                            "stringValue": "opentelemetry"
                        }
                    },
                    {
                        "key": "telemetry.sdk.language",
                        "value": {
                            "stringValue": "dotnet"
                        }
                    },
                    {
                        "key": "telemetry.sdk.version",
                        "value": {
                            "stringValue": "1.8.1"
                        }
                    }
                ]
            },
            "scopeLogs": [
                {
                    "scope": {
                        "name": "LoggingExample.Program"
                    },
                    "logRecords": [
                        {
                            "timeUnixNano": "1720176951757341200",
                            "observedTimeUnixNano": "1720176951757341200",
                            "severityNumber": 17,
                            "severityText": "Error",
                            "body": {
                                "stringValue": "Hello Universe"
                            },
                            "attributes": [
                                {
                                    "key": "exception.type",
                                    "value": {
                                        "stringValue": "InvalidOperationException"
                                    }
                                },
                                {
                                    "key": "exception.message",
                                    "value": {
                                        "stringValue": "Operation is not valid due to the current state of the object."
                                    }
                                },
                                {
                                    "key": "exception.stacktrace",
                                    "value": {
                                        "stringValue": "System.InvalidOperationException: Operation is not valid due to the current state of the object.\r\n   at LoggingExample.Program.CreateLogMessage(ILogger logger) in C:\\\\src\\LoggingExample\\CreateLogMessage.cs:line 11"
                                    }
                                },
                                {
                                    "key": "World",
                                    "value": {
                                        "stringValue": "Universe"
                                    }
                                },
                                {
                                    "key": "{OriginalFormat}",
                                    "value": {
                                        "stringValue": "Hello {World}"
                                    }
                                }
                            ],
                            "traceId": "",
                            "spanId": ""
                        }
                    ]
                }
            ]
        }
    ]
}

The converted ECS message is:

{
    "_index": ".ds-logs-LoggingExample-staging-2024.07.05-000001",
    "_id": "-W6MgpABF9Yym4iYb4mT",
    "_version": 1,
    "_score": 0,
    "_source": {
        "observer": {
            "hostname": "2630a8071eee",
            "type": "apm-server",
            "version": "8.13.4"
        },
        "agent": {
            "name": "opentelemetry/dotnet",
            "version": "1.8.1"
        },
        "@timestamp": "2024-07-05T10:59:19.037Z",
        "log": {
            "level": "Error"
        },
        "data_stream": {
            "namespace": "staging",
            "type": "logs",
            "dataset": "LoggingExample"
        },
        "service": {
            "node": {
                "name": "19d619b4-a623-45a4-97bb-62e59a14d7ef"
            },
            "environment": "Staging",
            "name": "LoggingExample",
            "language": {
                "name": "dotnet"
            }
        },
        "error": {
            "exception": [
                {
                    "handled": true,
                    "message": "Operation is not valid due to the current state of the object.",
                    "type": "InvalidOperationException"
                }
            ],
            "id": "d80e089e9f26956ba8280a475c2a2fe0",
            "stack_trace": "System.InvalidOperationException: Operation is not valid due to the current state of the object.\r\n   at LoggingExample.Program.CreateLogMessage(ILogger logger) in C:\\\\src\\LoggingExample\\CreateLogMessage.cs:line 11",
            "grouping_key": "b4fa1fdccaecf80e97197bb11377e589",
            "grouping_name": "Operation is not valid due to the current state of the object."
        },
        "event": {
            "severity": 17,
            "kind": "event",
            "type": "error"
        },
        "message": "Operation is not valid due to the current state of the object.",
        "labels": {
            "service_namespace": "LoggingExample",
            "{OriginalFormat}": "Hello {World}",
            "World": "Universe"
        },
        "timestamp": {
            "us": 1720177159037815
        }
    },
    "fields": {
        "error.grouping_key": [
            "b4fa1fdccaecf80e97197bb11377e589"
        ],
        "error.exception.type": [
            "InvalidOperationException"
        ],
        "service.node.name": [
            "19d619b4-a623-45a4-97bb-62e59a14d7ef"
        ],
        "service.language.name": [
            "dotnet"
        ],
        "service.node.name.text": [
            "19d619b4-a623-45a4-97bb-62e59a14d7ef"
        ],
        "agent.name.text": [
            "opentelemetry/dotnet"
        ],
        "log.level": [
            "Error"
        ],
        "agent.name": [
            "opentelemetry/dotnet"
        ],
        "labels.World": [
            "Universe"
        ],
        "event.kind": [
            "event"
        ],
        "event.severity": [
            17
        ],
        "service.environment": [
            "Staging"
        ],
        "service.name": [
            "LoggingExample"
        ],
        "data_stream.namespace": [
            "staging"
        ],
        "service.language.name.text": [
            "dotnet"
        ],
        "error.exception.handled": [
            true
        ],
        "message": [
            "Operation is not valid due to the current state of the object."
        ],
        "data_stream.type": [
            "logs"
        ],
        "observer.hostname": [
            "2630a8071eee"
        ],
        "timestamp.us": [
            1720177159037815
        ],
        "@timestamp": [
            "2024-07-05T10:59:19.037Z"
        ],
        "observer.type": [
            "apm-server"
        ],
        "observer.version": [
            "8.13.4"
        ],
        "service.name.text": [
            "LoggingExample"
        ],
        "data_stream.dataset": [
            "LoggingExample"
        ],
        "error.id": [
            "d80e089e9f26956ba8280a475c2a2fe0"
        ],
        "event.type": [
            "error"
        ],
        "agent.version": [
            "1.8.1"
        ],
        "error.grouping_name": [
            "Operation is not valid due to the current state of the object."
        ],
        "error.stack_trace": [
            "System.InvalidOperationException: Operation is not valid due to the current state of the object.\r\n   at LoggingExample.Program.CreateLogMessage(ILogger logger) in C:\\\\src\\LoggingExample\\CreateLogMessage.cs:line 11"
        ],
        "error.stack_trace.text": [
            "System.InvalidOperationException: Operation is not valid due to the current state of the object.\r\n   at LoggingExample.Program.CreateLogMessage(ILogger logger) in C:\\\\src\\LoggingExample\\CreateLogMessage.cs:line 11"
        ],
        "labels.service_namespace": [
            "LoggingExample"
        ],
        "error.exception.message": [
            "Operation is not valid due to the current state of the object."
        ],
        "labels.{OriginalFormat}": [
            "Hello {World}"
        ]
    }
}