The Hosted Grafana platform uses multiple clusters of machines to provide the highest possible availability and reliability, which means that both the source IP for a given instance and the set of IPs used by all instances can (and do) change quite regularly.
In general we advise strongly against relying on source IP address based access control, not only because of the issues with maintaining a list of allowed IP addresses but because there will be multiple Grafana instances on any given host which will necessarily share the same source IP and thus would also have access to your data source. In addition, the traffic between your Grafana instance and your data source would not be encrypted in transit.
Our standard recommendation is to place an nginx reverse proxy in front of your data source and use that to provide transport security with TLS and authentication either via HTTP basic auth or TLS client certificates, both of which are supported by Grafana.