metrics: fix issues reported by staticcheck (#20365)

metrics/counter_test.go

metrics/gauge_float64_test.go

metrics/gauge_test.go

metrics/histogram_test.go

metrics/influxdb/influxdb.go

metrics/json_test.go

metrics/librato/client.go

metrics/librato/librato.go

metrics/meter_test.go

metrics/registry_test.go

metrics/runtime_test.go

metrics/sample.go

metrics/sample_test.go

metrics/timer.go

metrics/timer_test.go