feat(option/internaloption): add new EmbeddableAdapter option

internal/kokoro/vet.sh

@@ -37,6 +37,7 @@ golint ./... 2>&1 | ( \
   grep -v "exported method MarshalStyle.JSONReader should have comment or be unexported" | \
   grep -v "UnmarshalJSON should have comment or be unexported" | \
   grep -v "MarshalJSON should have comment or be unexported" | \
+  grep -v ".Apply should have comment or be unexported" | \
   grep -vE "\.pb\.go:" || true) | tee /dev/stderr | (! read)
 
 staticcheck -go 1.9 ./... 2>&1 | ( \

option/internaloption/internaloption.go

@@ -134,3 +134,10 @@ type withCreds google.Credentials
 func (w *withCreds) Apply(o *internal.DialSettings) {
 	o.InternalCredentials = (*google.Credentials)(w)
 }
+
+// EmbeddableAdapter is a no-op option.ClientOption that allow libraries to
+// create their own client options by embedding this type into their own
+// client-specific option wrapper. See example for usage.
+type EmbeddableAdapter struct{}
+
+func (*EmbeddableAdapter) Apply(_ *internal.DialSettings) {}

option/internaloption/internaloption_external_test.go

@@ -0,0 +1,62 @@
+// Copyright 2022 Google LLC.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package internaloption_test
+
+import (
+	"context"
+	"fmt"
+
+	"google.golang.org/api/option"
+	"google.golang.org/api/option/internaloption"
+)
+
+type config struct {
+	i int
+}
+
+type clientSpecificOption interface {
+	option.ClientOption
+	ApplyOpt(*config)
+}
+
+func WithFavoriteNumber(i int) option.ClientOption {
+	return &withFavoriteNumber{i: i}
+}
+
+type withFavoriteNumber struct {
+	internaloption.EmbeddableAdapter
+	i int
+}
+
+func (w *withFavoriteNumber) ApplyOpt(c *config) {
+	c.i = w.i
+}
+
+type Foo struct {
+	i int
+}
+
+func NewFoo(ctx context.Context, opts ...option.ClientOption) (*Foo, error) {
+	var conf config
+	for _, opt := range opts {
+		if fooOpt, ok := opt.(clientSpecificOption); ok {
+			fooOpt.ApplyOpt(&conf)
+		}
+	}
+	// Pass options to internals for dailing. All client specific options will
+	// be no-ops.
+	return &Foo{i: conf.i}, nil
+}
+
+func (f *Foo) Number() int { return f.i }
+
+func ExampleEmbeddableAdapter() {
+	f, err := NewFoo(context.Background(), WithFavoriteNumber(42))
+	if err != nil {
+		// TODO: handle error
+	}
+	fmt.Println(f.Number())
+	// Output: 42
+}