OpenTelemetry UpDown Counter
https://opentelemetry.io/docs/specs/otel/metrics/api/#updowncounter
Gauge のようにも見えるが,実体としては non-monotonic な Sum
https://github.com/open-telemetry/opentelemetry-go/blob/b761af75f570d12ab99ae30af1333735a136f3c0/sdk/metric/pipeline.go?plain=1#L464-L467