[Feature Request]: AnthropicConfig open_telemetry option #3490

Is your feature request related to a problem? Please describe.

anthropic-sdk-python 1.14 added native OpenTelemetry tracing, on by default. With a global tracer provider registered, the SDK creates an anthropic.messages.create CLIENT span (scope com.anthropic.sdk.python) and sends a traceparent header. I confirmed this on 1.17.0 with an in-memory exporter: the span is nested under the current span, which with TelemetryMiddleware is ag2's own chat span, so it adds a child rather than duplicating.

Users can switch it off only with the ANTHROPIC_OPEN_TELEMETRY=false environment variable. AnthropicConfig does not expose the SDK's open_telemetry client option.

Describe the solution you'd like

Expose open_telemetry on AnthropicConfig (pass-through to the client) and mention the new SDK spans and the opt-out in the telemetry docs.

Additional context

Not checked: whether any existing ag2 test or user assertion depends on the number of spans per request.