Capture current thread in otel traces

For long-lived spans (the ones for streaming RPCs and `HttpPayload`) we should capture the thread used for each event / child span.

Start by locating the long-lived span implementations for streaming RPCs and HttpPayload, then trace how events and child spans are created and exported through OpenTelemetry. Done means each event or child span captures the thread used for it, with tests covering both long-lived span cases.

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