Fix API LogRecord serialization

[xrmx]

Description

This fixes the following example:

from opentelemetry._logs import get_logger_provider
from opentelemetry._logs import LogRecord as APILogRecord

logger = get_logger_provider().get_logger(__name__)

logger.emit(APILogRecord(body="Hello, World!", attributes={"foo": "bar"}))

In both console:

{
    "body": "Hello, World!",
    "severity_number": null,
    "severity_text": null,
    "attributes": {
        "foo": "bar"
    },
    "dropped_attributes": 0,
    "timestamp": null,
    "observed_timestamp": "2025-09-09T08:41:35.905784Z",
    "trace_id": "0x00000000000000000000000000000000",
    "span_id": "0x0000000000000000",
    "trace_flags": 0,
    "resource": {
        "attributes": {
            "telemetry.sdk.language": "python",
            "telemetry.sdk.name": "opentelemetry",
            "telemetry.sdk.version": "1.37.0.dev0",
            "service.name": "unknown_service"
        },
        "schema_url": ""
    },
    "event_name": ""
}

And otlp exporters:

2025-09-09T09:34:21.086+0200	info	ResourceLog #0
Resource SchemaURL: 
Resource attributes:
     -> telemetry.sdk.language: Str(python)
     -> telemetry.sdk.name: Str(opentelemetry)
     -> telemetry.sdk.version: Str(1.37.0.dev0)
     -> service.name: Str(unknown_service)
ScopeLogs #0
ScopeLogs SchemaURL: 
InstrumentationScope __main__ 
LogRecord #0
ObservedTimestamp: 2025-09-09 07:34:21.083054264 +0000 UTC
Timestamp: 1970-01-01 00:00:00 +0000 UTC
SeverityText: 
SeverityNumber: Unspecified(0)
Body: Str(Hello, World!)
Attributes:
     -> foo: Str(bar)
Trace ID: 
Span ID: 
Flags: 0
	{"otelcol.component.id": "debug", "otelcol.component.kind": "Exporter", "otelcol.signal": "logs"}

This is an alternative version of #4737 with one more fix and no api change.

Refs #4319

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• tox

Does This PR Require a Contrib Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[aabmass]

Other than Dylan's comments LGTM