Revert "Move exporter prometheus"

[codeboten]

Reverts #81

[alolita]

@codeboten My team had added the PRW exporter initially to the Python SDK (see https://aws.amazon.com/blogs/opensource/building-a-prometheus-remote-write-exporter-for-the-opentelemetry-python-sdk/).

Any reason why the code was removed from the project? Where is the code now?

[codeboten]

@alolita the code for the prometheus and prometheus remote write exporters was moved to the metrics branch for the python and python contrib repos:

https://github.com/open-telemetry/opentelemetry-python/tree/metrics/exporter/opentelemetry-exporter-prometheus
https://github.com/open-telemetry/opentelemetry-python-contrib/tree/metrics/exporter/opentelemetry-exporter-prometheus-remote-write

As these relied on the previous implementation of the metrics SDK, they were moved at the same time as that code to allow the tracing signal to be released as 1.0. See this issue open-telemetry/opentelemetry-python#2291 for a discussion of bringing the code back into main now that the metrics API/SDK are being merged into that branch.