Don't allow NodeReplacer on a node that has multiple selectors

At this level we have no way to know if those selectors will be on the
same node (combining them into a single downstream query -- e.g.
aggregation imposes that limitation). So instead we simply disallow
NodeReplace for anything with multiple selectors as children.

Fixes #456

pkg/proxystorage/proxy.go

@@ -208,6 +208,11 @@ func (p *ProxyStorage) NodeReplacer(ctx context.Context, s *parser.EvalStmt, nod
 		return ok
 	}
 
+	isVectorSelector := func(node parser.Node) bool {
+		_, ok := node.(*parser.VectorSelector)
+		return ok
+	}
+
 	// If we are a child of a subquery; we just skip replacement (since it already did a nodereplacer for those)
 	for _, n := range path {
 		if isSubQuery(n) {
@@ -219,8 +224,9 @@ func (p *ProxyStorage) NodeReplacer(ctx context.Context, s *parser.EvalStmt, nod
 	// rules around combining). We'll skip this node and let a lower layer take this on
 	aggFinder := &BooleanFinder{Func: isAgg}
 	offsetFinder := &OffsetFinder{}
+	vecFinder := &BooleanFinder{Func: isVectorSelector}
 
-	visitor := NewMultiVisitor([]parser.Visitor{aggFinder, offsetFinder})
+	visitor := NewMultiVisitor([]parser.Visitor{aggFinder, offsetFinder, vecFinder})
 
 	if _, err := parser.Walk(ctx, visitor, s, node, nil, nil); err != nil {
 		return nil, err
@@ -233,6 +239,13 @@ func (p *ProxyStorage) NodeReplacer(ctx context.Context, s *parser.EvalStmt, nod
 		}
 	}
 
+	// If there is more than 1 vector selector here and we are not a subquery
+	// we can't combine as we don't know if those 2 selectors will for-sure
+	// land on the same node
+	if vecFinder.Found > 1 && !isSubQuery(node) {
+		return nil, nil
+	}
+
 	// If the tree below us is not all the same offset, then we can't do anything below -- we'll need
 	// to wait until further in execution where they all match
 	var offset time.Duration