x/oauth2: apply the expires_in value returned by the server to Token.ExpiresIn

In typical usage, Token.Expiry alone is sufficient.
However, the ExpiresIn field was introduced in golang/go#61417.
Even when a server returns an expires_in value in
methods like Config.Exchange, only the Expiry field is updated,
leaving ExpiresIn unchanged, which can cause confusion.

This change ensures that the ExpiresIn field is properly updated
when the server provides an expires_in value.

merges golang#748 (golang#748)

Author: soh335

internal/token.go

@@ -50,6 +50,11 @@ type Token struct {
 	// mechanisms for that TokenSource will not be used.
 	Expiry time.Time
 
+	// ExpiresIn is the OAuth2 wire format "expires_in" field,
+	// which specifies how many seconds later the token expires,
+	// relative to an unknown time base approximately around "now".
+	ExpiresIn int64
+
 	// Raw optionally contains extra metadata from the server
 	// when updating a token.
 	Raw interface{}
@@ -295,6 +300,7 @@ func doTokenRoundTrip(ctx context.Context, req *http.Request) (*Token, error) {
 		expires, _ := strconv.Atoi(e)
 		if expires != 0 {
 			token.Expiry = time.Now().Add(time.Duration(expires) * time.Second)
+			token.ExpiresIn = int64(expires)
 		}
 	default:
 		var tj tokenJSON
@@ -312,6 +318,7 @@ func doTokenRoundTrip(ctx context.Context, req *http.Request) (*Token, error) {
 			TokenType:    tj.TokenType,
 			RefreshToken: tj.RefreshToken,
 			Expiry:       tj.expiry(),
+			ExpiresIn:    int64(tj.ExpiresIn),
 			Raw:          make(map[string]interface{}),
 		}
 		json.Unmarshal(body, &token.Raw) // no error checks for optional fields

token.go

@@ -163,6 +163,7 @@ func tokenFromInternal(t *internal.Token) *Token {
 		TokenType:    t.TokenType,
 		RefreshToken: t.RefreshToken,
 		Expiry:       t.Expiry,
+		ExpiresIn:    t.ExpiresIn,
 		raw:          t.Raw,
 	}
 }