Add otlp exemplar ingestion test

[SungJin1212]

Add an otlp exemplar ingestion test.

Which issue(s) this PR fixes:
Fixes #6235

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[SungJin1212]

@yeya24
I think an otlp metadata ingestion test is needed as well. Let me add it.

[yeya24]

@SungJin1212 I don't think OTLP protocol supports metadata as metadata is only a Prometheus thing

[SungJin1212]

@yeya24
I just found out the otlp ingestion of the metadata is not implemented yet. Does that mean that the implementation itself is not necessary?

[yeya24]

https://github.com/prometheus/prometheus/blob/main/storage/remote/write_handler.go#L527
If you look at the Prometheus OTLP handler implementation it only ingests timeseries.

https://opentelemetry.io/docs/specs/otlp/
Metadata is only part of Prometheus remote write protocol but not part of OTLP specification so we can skip

[SungJin1212]

@yeya24
I got it. Thank you!

[yeya24]

prometheus/prometheus#13439 @SungJin1212 We can cover metadata API after this upstream PR gets merged

[yeya24]

Thanks. Great work. Just some nits