Constant CHILD_SPAN_EARLY_NANOS 

const CHILD_SPAN_EARLY_NANOS: i64 = _; // 300_000_000_000i64

A child server span starts no earlier than 5 minutes before its client span (clock-skew allowance) and no later than 1 hour after it; the bounds keep the join windowed instead of pairing arbitrarily distant spans of a long-lived trace.