[Schema]Update exemplar field in metrics.mapping

[YANG-DB]

Which domain protocol is relevant for this schema ?

According to the discussion made here the exemplar should become plural instead of singular

What is the schema resource ?

Otel metrics schema

Describe the schema resource you are interested in update or create and attach a link to the schema resource

What is the schema change ?
Transform exemplar into exemplars including updating the samples and documentation

---

Do you have any additional context?

Add any other context or screenshots about the feature request here.