storage: third party dependency exception for github.com/cloudprober/cloudprober

[noahdietz]

Module: storage/internal/benchmarks
Usage(s): storage/internal/benchmarks/main.go

For the time being, it will be exempt from the third party dep check, but please do one of the following:

A. Remove the dependency by switching to a package own directly by Google, handwriting necessary functionality, or using stdlib
B. Justify the exception and ack the risks, maintaining the exception indefinitely

[noahdietz]

Exempting indefinitely via option B.

This is an internal only module that end users will not interact with and is module-separated from the actual client library. There shouldn't be a concern here.