Mimir rejects samples when exemplar is non-compliant

[gouthamve]

See

mimir/pkg/distributor/distributor.go

Lines 755 to 761 in 04e6b7f

	if err := validateExemplar(d.exemplarValidationMetrics, userID, ts.Labels, e); err != nil {
	// An exemplar validation error prevents ingesting samples
	// in the same series object. However because the current Prometheus
	// remote write implementation only populates one or the other,
	// there never will be any.
	return err
	}

We reject the entire sample if the exemplar attached is non-compliant (see: #7904). OTel sends empty exemplars by default and this means OTel users' data is being dropped regularly.