Fix logging context warnings due to common usage metrics setup (#14574)

`setup()` is run under the sentinel context manager, so we wrap the
initial update in a background process. Before this change, Synapse
would log two warnings on startup:
    Starting db txn 'count_daily_users' from sentinel context
    Starting db connection from sentinel context: metrics will be lost

Signed-off-by: Sean Quah <seanq@matrix.org>