Add Receiver for Prometheus

[MoizBhayani]

Adding receiver port for prometheus

[lakamsani]

This is to support open-telemetry#931 (comment). It will enable the OTel demo to work with the OTel operator for prometheus metrics.

https://github.com/open-telemetry/opentelemetry-helm-charts/blob/main/charts/opentelemetry-demo/examples/default/rendered/opentelemetry-collector/service.yaml#L42

[lakamsani]

@VineethReddy02 how does this look as a follow-up to our discussion on that enhancement? As this is just about making a port available, does it make a difference if we call it receiver_prometheus.go and not exporter_prometheus.go?

@MoizBhayani suggest changing this PR to draft while you add tests etc.