feat(googleapi): support setting arbitrary query parameters (#1123)

Fixes: #550

googleapi/googleapi.go

@@ -391,6 +391,14 @@ type CallOption interface {
 	Get() (key, value string)
 }
 
+// A MultiCallOption is an option argument to an API call and can be passed
+// anywhere a CallOption is accepted. It additionally supports returning a slice
+// of values for a given key.
+type MultiCallOption interface {
+	CallOption
+	GetMulti() (key string, value []string)
+}
+
 // QuotaUser returns a CallOption that will set the quota user for a call.
 // The quota user can be used by server-side applications to control accounting.
 // It can be an arbitrary string up to 40 characters, and will override UserIP
@@ -417,4 +425,24 @@ type traceTok string
 
 func (t traceTok) Get() (string, string) { return "trace", "token:" + string(t) }
 
+type queryParameter struct {
+	key    string
+	values []string
+}
+
+// QueryParameter allows setting the value(s) of an arbitrary key.
+func QueryParameter(key string, values ...string) CallOption {
+	return queryParameter{key: key, values: append([]string{}, values...)}
+}
+
+// Get will never actually be called -- GetMulti will.
+func (q queryParameter) Get() (string, string) {
+	return "", ""
+}
+
+// GetMulti returns the key and values values associated to that key.
+func (q queryParameter) GetMulti() (string, []string) {
+	return q.key, q.values
+}
+
 // TODO: Fields too

internal/gensupport/params.go

@@ -43,9 +43,15 @@ func (u URLParams) Encode() string {
 	return url.Values(u).Encode()
 }
 
-// SetOptions sets the URL params and any additional call options.
+// SetOptions sets the URL params and any additional `CallOption` or
+// `MultiCallOption` passed in.
 func SetOptions(u URLParams, opts ...googleapi.CallOption) {
 	for _, o := range opts {
+		m, ok := o.(googleapi.MultiCallOption)
+		if ok {
+			u.SetMulti(m.GetMulti())
+			continue
+		}
 		u.Set(o.Get())
 	}
 }

internal/gensupport/params_test.go

@@ -0,0 +1,20 @@
+// Copyright 2021 Google LLC. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package gensupport
+
+import (
+	"testing"
+
+	"google.golang.org/api/googleapi"
+)
+
+func TestSetOptionsGetMulti(t *testing.T) {
+	co := googleapi.QueryParameter("key", "foo", "bar")
+	urlParams := make(URLParams)
+	SetOptions(urlParams, co)
+	if got, want := urlParams.Encode(), "key=foo&key=bar"; got != want {
+		t.Fatalf("URLParams.Encode() = %q, want %q", got, want)
+	}
+}