Trace context
traceparent HTTP header. traceId (128-bit) + spanId (64-bit) + flags. Passed through service chain.
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Baggage
Optional key-value context. Not for high-cardinality. user_id, request_type. Propagated with trace context.
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Sampling strategies
Head: decide at trace start. Simple + biased. Tail: decide after completion. Keep interesting (errors, slow). Cost-benefit.