[receiver/prometheusreceiver] Add exemplars to prometheus receiver

[aishyandapalli]

Description: Added a support to acknowledge exemplars from prometheus receiver and attach the exemplars to otel metric object.

Adding a feature Currently, there is no support for exemplars in prometheus receiver. Added functionality to address this

Link to tracking Issue: https://github.com/open-telemetry/opentelemetry-collector-contrib/issues/8353

Testing: Used prometheusremotewrite to send data with exemplars and verified if we are able to visualize exemplars data.