opentelemetry-exporter-prometheus: restore package

[codeboten]

Description

The Prometheus exporter was moved into the metrics branch prior to releasing 1.0 for tracing. Now that the metrics API and SDK are further along, we can bring back the exporter and update it to use the new methods. The original code for this PR comes from https://github.com/open-telemetry/opentelemetry-python/tree/metrics/exporter/opentelemetry-exporter-prometheus

Fixes #2291

[srikanthccv]

Added some initial review comments

[srikanthccv]

many moving parts here with latest changes. @codeboten would you mind adding some ascii diagram of the pipeline? that would really help me to get the better idea of whole thing.

One question I have is if there are multiple exporter configured ex (otlp, prometheus), is there a chance of discrepancies in metric output because of different collection cycles? each metric reader has separate state so this shouldn't be a problem.

[codeboten]

many moving parts here with latest changes. @codeboten would you mind adding some ascii diagram of the pipeline? that would really help me to get the better idea of whole thing.

@srikanthccv Sure here's an attempt, please take a look:

 ------------------
| PullMetricReader | -- registers collect callback
 ------------------         |        
                            |
         --------------------------                          --------------------
        | PrometheusMetricExporter | <--- implementation of | PullMetricExporter |
         --------------------------                          --------------------
                    /\                                       
             ------------------
            | _CustomCollector | <---- registered with prometheus client library
             ------------------
                    /\
 -------------------------------------------
| ScrapeTarget (e.g localhost:8000/metrics) |
 -------------------------------------------

[codeboten]

@srikanthccv @aabmass @ocelotl please take a look at the latest change, i've removed the added interfaces, which i think makes it a lot cleaner on the implementation side... still undecided if this makes things better for the users though.