Passing custom fields during user onboarding with metadata

[mffap]

Preflight Checklist

• I could not find a solution in the existing issues, docs, nor discussions
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Describe the docs your are missing or that are wrong

As a developer, I want to build a custom user onboarding/register and save additional fields, so I can use those fields in my app.

We provide a doc on how to build a user onboarding and on how to use user metadata. But we don't cross reference.

The obvious two solutions to handling cases like that are:

1/ Backend: One request to create the user in ZITADEL and one to your backend/app to save the data
2/ Metadata: Create the user in ZITADEL, add the extra fields as metadata on the user; retrieval: a/ in the token b/ api request c/ custom claims
(3) #6433 tbd.

Those cases should probably be mentioned on the docs page.

Additional Context

No response

[yordis]

Would there be any way to pass Transient Metadata? Related to ory/kratos#3102

Such Transient Metadata is not attached to the resource but to the workflow so that we can consume Events with such metadata to make a decision.

They are not attached to the resources because the validity and value of such data are for a given workflow. Having Processors around Zitadel understand WHEN such data was true is difficult since we external Processors don't own the workflow. Timing issues of when to read, write, update, or delete such information are extremely expensive and easy to mess up.