Change default sampling rate from 100% to 0%

This changes the default parent-based trace sampling rate from
100% to 0%. Due to the high QPS etcd can handle, having 100% trace
sampling leads to very high resource usage. Defaulting to 0% means
that only already-sampled traces will be sampled in etcd.

Fixes etcd-io#14310

server/embed/etcd.go

@@ -849,6 +849,7 @@ func (e *Etcd) setupTracing(ctx context.Context) (exporter tracesdk.SpanExporter
 			tracesdk.NewTracerProvider(
 				tracesdk.WithBatcher(exporter),
 				tracesdk.WithResource(res),
+				tracesdk.WithSampler(tracesdk.ParentBased(tracesdk.NeverSample())),
 			),
 		),
 	)