Support Exemplars in OTLP metric exporters

[dashpole]

Problem Statement

When using exemplars, I should be able to send them over OTLP. I tried sending them to a collector using OTLP http and gRPC, and was unable to do so. It looks like this isn't implemented yet in the OTLP metric exporters:

For example, it doesn't look like its set in the grpc exporter for histograms:

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

Lines 188 to 196 in dd5d054

	hdp := &mpb.HistogramDataPoint{
	Attributes: AttrIter(dPt.Attributes.Iter()),
	StartTimeUnixNano: timeUnixNano(dPt.StartTime),
	TimeUnixNano: timeUnixNano(dPt.Time),
	Count: dPt.Count,
	Sum: &sum,
	BucketCounts: dPt.BucketCounts,
	ExplicitBounds: dPt.Bounds,
	}

Proposed Solution

Implement exemplars in OTLP exporters.