For now, what I did was to use templates.

        source: z_full_json
        # Truncate to 64KB (adjust size as needed)
        template: '{{ if gt (len .Value) 65536 }}{{ slice .Value 0 65536 }}...TRUNCATED{{ else }}{{ .Value }}{{ end }}'

This way, promtail will truncate any huge json that loki might reject. This way, I am still able to get most of my json label values for each log and only miss a few.