* Use the impersonator id to prevent these tokens from counting against
the impersonated user when limiting the users' total of access tokens.
* When connecting using an access token with an impersonator add the
impersonator as a field to the tracing span that wraps the task for that
connection.
* Disallow impersonating users via the admin API token in production,
because when using the admin API token, we aren't able to identify the
impersonator.