Conditional on context.isSampled in Span operator=().

In operator= overload we need to check if the current span which is being
overwritten is eligble for serialization. If so we must send to the tracer
via the g_tracer->trace(*this) call. Previously we were incorrectly checking
the function argument o's context for sampling. This resulted in loss traces
for the NAPI:readVersionBatcher spans.

fdbclient/Tracing.actor.cpp

@@ -441,7 +441,7 @@ void openTracer(TracerType type) {
 ITracer::~ITracer() {}
 
 Span& Span::operator=(Span&& o) {
-	if (begin > 0.0 && o.context.isSampled() > 0) {
+	if (begin > 0.0 && context.isSampled()) {
 		end = g_network->now();
 		g_tracer->trace(*this);
 	}