correctly parse otlp HistogramDataPoint without distribution of values

[niecore]

The Metrics Data Model for Histograms defines a format for histogram data points that does not contain value distribution (see also https://github.com/open-telemetry/opentelemetry-proto/blob/v0.9.0/opentelemetry/proto/metrics/v1/metrics.proto#L498). In that case only count and sum should be transmitted. This HistogramDataPoint can currently not be parsed by the elastic apm server: https://github.com/elastic/apm-data/blob/main/input/otlp/metrics.go#L367

I discovered this problem while transmitting micrometer timers via the otel micrometer instrumentation. This histograms do not contain any buckets by default.

[simitt]

@raultorrecilla and @akhileshpok for input on priority.

[ericywl]

@simitt @raultorrecilla May I know what would be the expected resolution here? Do we simply allow parsing histograms without buckets?

[kruskall]

According to the spec, buckets are optional so I'd say yes: https://opentelemetry.io/docs/specs/otel/metrics/data-model/#histogram

[ericywl]

Thanks, I will look into this for next iteration.