Golang opentelemetry sdk doesn't support delta aggregations with PrecomputedSum

[veqryn]

See notes here: open-telemetry/opentelemetry-go#2868

The current example has a temporality selector that forces CounterObserver to use delta temporality, which causes the opentelemetry sdk to throw errors and not export the metric.

[TylerHelmuth]

I agree. If I add an CounterObserver to the metrics.go example it will print out "cumulative to delta not implemented" and not export the metric.

This is a limitation with the Go Metrics SDK, which isn't stable yet. Being able to set delta temporality for Async Counters is part of the otel specification, though, so I would expect the capability to be added before a stable Go metrics SDK release.

For this example, if we switch CounterObserverInstrumentKind to delta then New Relic will treat the data as a Gauge, which is slightly misleading.

[TylerHelmuth]

Guess that is better than an error though. I will update the temporality selector so that an error is not thrown. When the Go SDK supports delta temporality for these instruments in the future I will add it back.

[veqryn]

I can make a PR if you'd like