implements json stage metrics

pkg/logentry/counters.go

pkg/logentry/metric/counters.go

pkg/logentry/metric/counters_test.go

pkg/logentry/metric/entries.go

pkg/logentry/metric/gauges.go

pkg/logentry/metric/histograms.go

pkg/logentry/metric/metricvec.go

pkg/logentry/stages/extensions.go

pkg/logentry/stages/json.go

pkg/logentry/stages/json_test.go

pkg/logentry/stages/metrics.go

pkg/logentry/stages/stage.go

pkg/logentry/stages/util_test.go