oauth2: sync Token.ExpiresIn from internal Token

The internal.Token type is intended to mirror the public Token type.
Unmarshalling is sometimes done on the internal token.
Sync the field addition, and update the conversion.

Updates #61417
Fixes golang/oauth2#746

Change-Id: I55818810394adf743afb8d6d3be477a425c233b5
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/666815
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Matt Hickford <matt.hickford@gmail.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>

internal/token.go

@@ -49,6 +49,13 @@ 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".
+	// It is the application's responsibility to populate
+	// `Expiry` from `ExpiresIn` when required.
+	ExpiresIn int64 `json:"expires_in,omitempty"`
+
 	// Raw optionally contains extra metadata from the server
 	// when updating a token.
 	Raw interface{}
@@ -311,6 +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,
 	}
 }