decode error when sending span.context.destination.port as string

**Describe the bug**:

According to the specification, span.context.destination.port must be a number.

As part of the ops KPI review we've been investigating the following errors in ecs logs:

```
decode error: data read error: v2.spanRoot.Span: v2.span.Context: v2.spanContext.Destination: v2.spanContextDestination.Port: readUint64: unexpected character: �, error found in #10 byte of ...| "port": "9243", "se|..., bigger context ...|redacted", "port": "9243", "service": {"resource": "elasticsearch/aaaa|...
```

```
decode error: data read error: v2.spanRoot.Span: v2.span.Context: v2.spanContext.Destination: v2.spanContextDestination.Port: readUint64: unexpected character: �, error found in #10 byte of ...| "port": "9|..., bigger context ...|: "redacted", "port": "9|...
```

`"port": "9243"` is not valid format and the apm-server will fail to ingest those transactions.

**To Reproduce**

I don't have steps to reproduce, this is part of the ops KPI review and was observed in the ecs logs.

**Environment (please complete the following information)**

Most of the errors are coming from the following user agent: apm-agent-python/6.18.0

This happens on multiple apm-server versions. Both 7.x and 8.x, including the latest 8.9.0.

**Additional context**

none

No se identifica ningún archivo fuente ni ninguna prueba. Empieza rastreando cómo el Python agent serializa span.context.destination.port en la versión 6.18.0; después, compara el payload emitido con la especificación e investiga un caso reproducible para los datos ECS indicados. Se considera terminado cuando se emiten valores de puerto numéricos y una prueba de regresión cubre el fallo.

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