Get count from Histogram using View

[KevinLiLu]

I am using metric views to collect and export metrics. I have a Histogram which is tracking request latencies, and I want to export the total request count as a separate metric.

The spec mentions this should be possible via customizing the aggregation for the view (link):

A View provides SDK users with the flexibility to customize the metrics that are output by the SDK. Here are some examples when a View might be needed:
...
Customize the aggregation - if the default aggregation associated with the Instrument does not meet the needs of the user. For example, an HTTP client library might expose HTTP client request duration as Histogram by default, but the application developer might only want the total count of outgoing requests.

How would this work in the Java SDK? I don't see a count aggregation in Aggregation.java.

[jkwatson]

Unfortunately, there isn't a Count aggregation specified currently [https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md?plain=1#L558], and we don't currently support custom aggregations in the standard SDK. If this is something you need, I recommend opening up an issue in the specification to add a new aggregation type.