[logger] Trim base paths from file and function names

[manugarg]

A log line like the following:

{"time":"2023-08-22T18:14:07.77109-07:00","level":"INFO","source":{"function":"github.com/cloudprober/cloudprober/prober.(*Prober).addProbe","file":"/home/runner/work/cloudprober/cloudprober/prober/prober.go","line":112},"msg":"Creating a HTTP probe: api_server","system":"cloudprober","component":"global"}

changes to:

{"time":"2023-08-22T18:14:07.77109-07:00","level":"INFO","source":{"function":"prober.(*Prober).addProbe","file":"prober/prober.go","line":112},"msg":"Creating a HTTP probe: api_server","system":"cloudprober","component":"global"}