Add exemplars coverage in OTLP integration test

[yeya24]

Is your feature request related to a problem? Please describe.

The existing OTLP push integration test covers the ingestion of both series and histograms. However, we didn't cover exemplar data type.
https://github.com/cortexproject/cortex/blob/master/integration/otlp_test.go

Describe the solution you'd like

Ingest exemplars data in the integration test and make sure the exemplars can be ingested successfully via OTLP endpoint.

[SungJin1212]

@yeya24
Please assign me!