Consider adding summary support to metricdata

[dashpole]

#4571 is currently blocked on a spec decision. In the meantime, we can at least look at what adding summary support would look like in the Go SDK with a prototype.

[dashpole]

See #4600 for what adding the summary type to the data model and bridge would look like.

It does not require introducing the Summary type in the SDK (e.g. through views), since the Aggregation used in the view's Stream differs from metricdata.Aggregation.

[dashpole]

Current exporters, have switch statements over Data.(type) of the aggregation, and either error on unknown values, or ignore them. So this would be a relatively safe change. We would want to follow-up by introducing summary support to OTLP and Prometheus exporters.

opentelemetry-go/exporters/prometheus/exporter.go

Lines 217 to 230 in 68241af

	switch v := m.Data.(type) {
	case metricdata.Histogram[int64]:
	addHistogramMetric(ch, v, m, keys, values, name)
	case metricdata.Histogram[float64]:
	addHistogramMetric(ch, v, m, keys, values, name)
	case metricdata.Sum[int64]:
	addSumMetric(ch, v, m, keys, values, name)
	case metricdata.Sum[float64]:
	addSumMetric(ch, v, m, keys, values, name)
	case metricdata.Gauge[int64]:
	addGaugeMetric(ch, v, m, keys, values, name)
	case metricdata.Gauge[float64]:
	addGaugeMetric(ch, v, m, keys, values, name)
	}

opentelemetry-go/exporters/otlp/otlpmetric/otlpmetricgrpc/internal/transform/metricdata.go

Lines 95 to 114 in 68241af

	switch a := m.Data.(type) {
	case metricdata.Gauge[int64]:
	out.Data = Gauge[int64](a)
	case metricdata.Gauge[float64]:
	out.Data = Gauge[float64](a)
	case metricdata.Sum[int64]:
	out.Data, err = Sum[int64](a)
	case metricdata.Sum[float64]:
	out.Data, err = Sum[float64](a)
	case metricdata.Histogram[int64]:
	out.Data, err = Histogram(a)
	case metricdata.Histogram[float64]:
	out.Data, err = Histogram(a)
	case metricdata.ExponentialHistogram[int64]:
	out.Data, err = ExponentialHistogram(a)
	case metricdata.ExponentialHistogram[float64]:
	out.Data, err = ExponentialHistogram(a)
	default:
	return out, fmt.Errorf("%w: %T", errUnknownAggregation, a)
	}

[dashpole]

We discussed this at the SIG meeting yesterday, and are going to move forward with adding this to the metricdata package.