Making the metrics variables for GetURLs protected

[michaeldinzinger]

Hello,

in an service implementation, which inherits from the abstract implementation of the FrontierService in this repository, we want to override the GetURLs method. Besides that, we want to use furtheron use the Prometheus metrics, which help to monitor the Frontier service and, among others, the number and latency of GetURLs calls. Therefore, it is necessary to make the three corresponding variables in AbstractFrontierService.java protected instead of private. Thank you!

[jnioche]

Good idea, thanks @michaeldinzinger