This indicates an attack attempt against an Information Disclosure vulnerability in Grafana.
A remote, authenticated attacker can exploit this vulnerability by creating a crafted dashboard panel then requesting a static render of the panel. Successful exploitation results in the disclosure of arbitrary file contents from the target server.
Grafana Labs Grafana 4.x prior to 4.6.5
Grafana Labs Grafana 5.x prior to 5.3.3
Information Disclosure: Remote attackers can gain sensitive information from vulnerable systems.
Refer to the vendor supplied advisory for updates: