tracing: fix OpenTracing traces across nodes

"Shadow traces" (OpenTracing) were broken at node boundaries because the
server shadow span was not using the remote span context.
I believe this was broken in cockroachdb#56109.

Fixes cockroachdb#62702

Release note (bug fix): OpenTracing traces work correctly again across
nodes. The client and server spans for RPCs are once again part of the
same trace instead of the server being a root span erroneously.

pkg/util/tracing/tracer.go

@@ -328,6 +328,8 @@ func (t *Tracer) startSpanGeneric(
 			var shadowCtx opentracing.SpanContext
 			if opts.Parent != nil && opts.Parent.i.ot.shadowSpan != nil {
 				shadowCtx = opts.Parent.i.ot.shadowSpan.Context()
+			} else if opts.RemoteParent != nil && opts.RemoteParent.shadowCtx != nil {
+				shadowCtx = opts.RemoteParent.shadowCtx
 			}
 			ot = makeShadowSpan(shadowTr, shadowCtx, opts.RefType, opName, startTime)
 			// If LogTags are given, pass them as tags to the shadow span.