Ensure metric synapse_user_logins_total always has a auth_provider

This issue has been migrated from [#9972](https://github.com/matrix-org/synapse/issues/9972).

---

When evaluating alerts against synapse_user_logins rate, we have an auth_provider set for the cases where password based auth is not used; but we don't have one for when normal password-based auth is used:
```
{auth_provider="oidc-apple", guest="False", instance="matrix.org"}
{auth_provider="oidc-facebook", guest="False", instance="matrix.org"}
{auth_provider="oidc-github", guest="False", instance="matrix.org"}
{auth_provider="oidc-gitlab", guest="False", instance="matrix.org"}
{auth_provider="oidc-google", guest="False", instance="matrix.org"}
{guest="False", instance="matrix.org"}
```

Consider allocating an auth_provider value for the standard password based auth, allowing all metrics to have a tag.

This isn't particularly urgent; currently alerts will just say that the auth provider "" is failing, rather than being more specific, but it feels like a good improvement.