Force flush traces at_exit

[delner]

Adds automatic span flushing on program exit to prevent data loss from buffered traces.

Problem

The OpenTelemetry BatchSpanProcessor buffers spans before export. If a Ruby process exits before the buffer flushes, those spans are lost silently. This is especially common in short-lived scripts or CLI tools.

Solution

Register an at_exit hook that calls force_flush on the tracer provider, ensuring all buffered spans are exported before the process terminates.

Configuration

Variable	Default	Description
BRAINTRUST_FLUSH_ON_EXIT	true	Set to false to disable automatic flushing

Disabling may be useful in long-running processes where graceful shutdown is handled separately, or in test environments where the overhead is undesirable.