convert custom attribute to string

[artpaul]

additional attributes may be of any type, so we should convert value to string instead of read it as a string type.

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[KalleOlaviNiemitalo]

Having the map contain JSON values encoded as strings was suggested in #1736 (comment).

This is a breaking change. Please add or update tests for non-string values in custom attributes.

[martin-g]

1. There is no JIRA ticket
2. The PR template is completely ignored