cleaning up remaining TODO's, adding tests

pkg/logentry/stages/configs.go

pkg/logentry/stages/extensions.go

pkg/logentry/stages/extensions_test.go

pkg/logentry/stages/json.go

pkg/logentry/stages/json_test.go

pkg/logentry/stages/labels.go

pkg/logentry/stages/labels_test.go

pkg/logentry/stages/match.go

pkg/logentry/stages/match_test.go

pkg/logentry/stages/metrics.go

pkg/logentry/stages/metrics_test.go

pkg/logentry/stages/output.go

pkg/logentry/stages/output_test.go

pkg/logentry/stages/pipeline.go

pkg/logentry/stages/pipeline_test.go

pkg/logentry/stages/regex.go

pkg/logentry/stages/regex_test.go

pkg/logentry/stages/stage.go

pkg/logentry/stages/timestamp.go

pkg/logentry/stages/timestamp_test.go