[Prometheus] Split up projects based on hosting mechanism.

[Yun-Ting]

Fixes #3057.

Changes

The updated project setup is as follows:

• OpenTelemetry.Exporter.Prometheus.Shared contains the PrometheusExporter, PrometheusExporterOptions and serialization methods for all the shared classes to both AspNetCore and HttpListener projects. This project is not expected to be consumed directly by the end users. It serves as a clean way to group shared code for the below 2 projects for semantic reasons. As a result, I've deleted the Implementation folder that was in OpenTelemetry.Exporter.Prometheus project, and moved out the shared files which was previously in the folder to be immediate children of the project.

• OpenTelemetry.Exporter.Prometheus.HttpListener contains the PrometheusHttpListener and its options class. It aims to provide end users a quick setup experience for using OTel metrics with Prometheus Exporter locally. Please refer to readme for more details.

• OpenTelemetry.Exporter.Prometheus.AspNetCore contains the middleware and its extensions for registering. This is for users trying to run PrometheusExporter in production environment. Refer to readme for details.

TODO:
Currently, OpenTelemetry.Exporter.Prometheus.AspNetCore.Tests, OpenTelemetry.Exporter.Prometheus.HttpListener.Tests, and OpenTelemetry.Exporter.Prometheus.Shared.Tests are green. I'll be working on fixing the CIs (not sure if I broke other tests with bad merge.)

For significant contributions please make sure you have completed the following items:

• Appropriate CHANGELOG.md updated for non-trivial changes
• Design discussion issue #
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #3375 (dec7d9d) into main (f4dddbe) will decrease coverage by 0.04%.
The diff coverage is 93.54%.

❗ Current head dec7d9d differs from pull request most recent head be87d44. Consider uploading reports for the commit be87d44 to get more accurate results

@@            Coverage Diff             @@
##             main    #3375      +/-   ##
==========================================
- Coverage   86.25%   86.20%   -0.05%     
==========================================
  Files         258      272      +14     
  Lines        9269     9519     +250     
==========================================
+ Hits         7995     8206     +211     
- Misses       1274     1313      +39     

Impacted Files	Coverage Δ
.../PrometheusExporterApplicationBuilderExtensions.cs	100.00% <ø> (ø)
...rometheusExporterEndpointRouteBuilderExtensions.cs	100.00% <ø> (ø)
...metheus.AspNetCore/PrometheusExporterMiddleware.cs	67.85% <ø> (ø)
...us/Implementation/PrometheusExporterEventSource.cs	27.77% <ø> (+11.11%)	⬆️
...elemetry.Exporter.Prometheus/PrometheusExporter.cs	100.00% <ø> (+9.09%)	⬆️
...y.Exporter.Prometheus/PrometheusExporterOptions.cs	57.14% <ø> (-27.07%)	⬇️
....Prometheus.HttpListener/PrometheusHttpListener.cs	82.35% <90.00%> (ø)
...heus.HttpListener/PrometheusHttpListenerOptions.cs	100.00% <100.00%> (ø)
...tp/Implementation/HttpHandlerDiagnosticListener.cs	69.31% <0.00%> (-3.86%)	⬇️
...ter.ZPages/Implementation/ZPagesActivityTracker.cs	97.14% <0.00%> (-2.86%)	⬇️
... and 30 more

[cijothomas]

@Yun-Ting Could you update the description with the final list of packages and their connections? It is not clear what would happend to the package OpenTelemetry.Exporter.Prometheus... Do we continue to publish it? Who is it meant for? Can one use it directly or its not meant for end user consumption?

[cijothomas]

Need to see description of the overall package architecture and dependencies, and the expected/proposed end user experience.

[Yun-Ting]

Need to see description of the overall package architecture and dependencies, and the expected/proposed end user experience.

It depends on whether we think there will be use cases for extending directly on OpenTelemetry.Exporter.Prometheus project (not using the middleware or the listener.)
I'm currently leaning towards on not exposing the OpenTelemetry.Exporter.Prometheus itself as a nuget and in the OpenTelemetry.Exporter.Prometheus.AspNetCore, and OpenTelemetry.Exporter.Prometheus.HttpListener, referencing the shared files as links.
What is your take @cijothomas ?

[cijothomas]

Need to see description of the overall package architecture and dependencies, and the expected/proposed end user experience.

It depends on whether we think there will be use cases for extending directly on OpenTelemetry.Exporter.Prometheus project (not using the middleware or the listener.) I'm currently leaning towards on not exposing the OpenTelemetry.Exporter.Prometheus itself as a nuget and in the OpenTelemetry.Exporter.Prometheus.AspNetCore, and OpenTelemetry.Exporter.Prometheus.HttpListener, referencing the shared files as links. What is your take @cijothomas ?

#3057 - the proposal from the issue is good.

[alanwest]

Suggested change

	private readonly System.Net.HttpListener httpListener = new();
	private readonly HttpListener httpListener = new();

[Yun-Ting]

Will get this waring without using System.Net for it :(.
What are the suggested ways to not spelling out the entire namespace for System.Net.HttpListener while not changing the PrometheusHttpListener namespace?

[Yun-Ting]

@Yun-Ting Could you update the description with the final list of packages and their connections? It is not clear what would happend to the package OpenTelemetry.Exporter.Prometheus... Do we continue to publish it? Who is it meant for? Can one use it directly or its not meant for end user consumption?

@cijothomas , I've updated the PR description to answer these questions. I don't think we'll need to publish OpenTelemetry.Exporter.Prometheus.Shared as a nuget package.
We will have 2 separate nuget packages, which are OpenTelemetry.Exporter.Prometheus.HttpListener and OpenTelemetry.Exporter.Prometheus.AspNetCore.

[github-actions]

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[cijothomas]

Where is PrometheusExporterOptions class defined? Why we need two separate options class?

[Yun-Ting]

PrometheusExporterOptions class in the OpenTelemetry.Exporter.Prometheus.Shared project and is meant to contain the fields that were shared by Listener and AspNetCore.

[cijothomas]

this'd be incorrect, as the link takes you to a console example, where this readme.md is specifically targetting asp.net core apps

[cijothomas]

See comments. Very close, but some refinements needed in the shared class usage, and public API

[Yun-Ting]

Closing this PR - please review the new #3430 instead. Thanks!