Can you please clarify which part of the metric has the 20k limitation? The limits on various parts of the metric are documented at
https://cloud.google.com/monitoring/quotas. In particular, the metric can have up to 30 labels, each of which can be up to 1k, making the total size of the metric point about 30k.
For larger data, you can send it to the logging API, though I'm not sure how you would hook it into Grafana. Log entries can be up to 256k in size, as per
https://cloud.google.com/logging/quotas. For even larger data, it's recommended that you upload it to another storage medium (e.g., GCS) and log a pointer.
It would help us to see an example of what you're trying to do…
Igor