OauthAuthenticator requires expires_in in response

When using OauthAuthenticator with an oauth2 endpoint that responds without the expires_in field we get the following error:

File "/.../site-packages/singer_sdk/authenticators.py", line 432, in update_access_token
    self.expires_in = token_json["expires_in"]
KeyError: 'expires_in'

https://gitlab.com/meltano/sdk/-/blob/8f8ee44e043c83917d3fe63ec646a13bad4014bf/singer_sdk/authenticators.py#L432

Looking at an OAuth2 spec, rfc6749 it looks like the expires_in field is RECOMMENDED but not REQUIRED. Does it seem sensible to extend OauthAuthenticator to support APIs which respond without an expires_in field?