Metrics don't work on .NET Core 3.1

[dpk83]

Bug Report

Metrics don't work on .NET Core 3.1 with latest OpenTelemetry RC builds
Version: 1.4.0-rc.4

Runtime version (e.g. netcoreapp3.1

Symptom

Using OpenTelemetry SDK to record metrics throws exception

What is the expected behavior?
This document #3767 mentions that it can technically be consumed on Net core 3.1 but that's not true.

What did you expect to see?
We still a bunch of services which are on 3.1 and the plan to upgrade to NET6 and higher is still an year away due to various factors. We expected OpenTelemetry to support 3.1 for one more year.

What is the actual behavior?
It completely breaks on 3.1

Reproduce

Repro is straightforward, just create a self-contained project to emit metrics running on .NET core 3.1

We will close this issue if:

• The repro project you share with us is complex. We can't investigate custom
  projects, so don't point us to such, please.
• If we can not reproduce the behavior you're reporting.

Additional Context

Add any other context about the problem here.

[reyang]

I don't think OpenTelemetry should continue to support .NET Core 3.1 since that's a deprecated version of runtime which will not even receive security hotfix. https://dotnet.microsoft.com/en-us/platform/support/policy/dotnet-core

The supported runtime versions and the strategy is clearly captured here https://github.com/open-telemetry/opentelemetry-dotnet#supported-net-versions.

[dpk83]

The document/annoucement then should be updated clearly to reflect the same. Currently it has wrong information

OpenTelemetry .NET can technically be consumed by projects targeting out-of-support frameworks like .NET Core 3.1 and even as far back as .NET Core 2.0.

[reyang]

I agree that the document should be clear. Where is the document?

[dpk83]

#3767

[alanwest]

We still do not plan to support .NET Core 3.1. We do not run tests against any out-of-support frameworks.

I agree that the document should be clear. Where is the document?

We will update the document to make this more clear.

[GeoK]

@alanwest @reyang What is the recommendation for library authors who ship multi-target SDKs (including netstandard2.0)?
Is there a way to ensure that metrics would get recorded in case that developers are using netcoreapp3.1?

[cijothomas]

@GeoK Not sure how is this any different for library authors who ship multi-target sdks.... If your app is ultimately run on unsupported .NET runtime, then Otel will not work.

[cijothomas]

Is there a way to ensure that metrics would get recorded in case that developers are using netcoreapp3.1?

No, we dont test in .NET Core 3.1, and this issue already proves we dont work in .NET Core 3.1