Add structured logging

[nikolaik]

Hi there, this adds structured logging using zap. You can change formats using --log_format Valid formats are json and plain.

Example run

$ go run main.go --github_token asdf --log_format=plain
2022-10-14T13:08:39.895+0200	INFO	metrics/metrics.go:85	authenticating with Github Token
2022-10-14T13:08:39.895+0200	INFO	server/server.go:33	exporter listening on 0.0.0.0:9999

$ go run main.go --github_token asdf
{"level":"info","ts":1665745733.755991,"caller":"metrics/metrics.go:85","msg":"authenticating with Github Token"}
{"level":"info","ts":1665745733.756206,"caller":"server/server.go:33","msg":"exporter listening on 0.0.0.0:9999"}

Open to changing anything about the implementation. For instance, does it make sense to be able to toggle structured/unstructured using a flag?

Note that pkg/metrics/get_runners_enterprise_from_github.go had CRLF line endings, so i converted those to LF.