Get all values entered into a text attribute

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Hi,
I have a 'text' attribute, 'birth_year', and users can enter any number in the field. I intend having a list of all the values - years - that inserted so far. Is there a way to retrieve them by C5 API?
I believe there is similar functionality for the 'select' attribute which is 'getOptionUsageArray()' method, but I couldn't find the one for text/number attributes.