in src/OpenTelemetryEngine/Traces/ActivityExtensions.cs [19:26]
static ActivityExtensions()
{
BindingFlags flags = BindingFlags.NonPublic | BindingFlags.Instance;
s_setSpanId = typeof(Activity).GetField("_spanId", flags)?.CreateSetter<Activity, string>();
s_setId = typeof(Activity).GetField("_id", flags)?.CreateSetter<Activity, string>();
s_setRootId = typeof(Activity).GetField("_rootId", flags)?.CreateSetter<Activity, string>();
s_setTraceId = typeof(Activity).GetField("_traceId", flags)?.CreateSetter<Activity, string>();
}