Prometheus exporter should return empty body with 200 code not 204 when no metrics

[kriswuollett]

Bug Report

Not sure if exactly the same as mentioned in #2400 since that mentions a part of the system not being ready, but I think I encountered the 204 issue when simply no metrics had been populated yet. If this is the same issue, IMO keeping this one to track this specific issue outside of a catch-all would be nice.

For the case when there are no metrics, the Prometheus export should return an empty body with 200 status code, not "204 Empty Content", because the Prometheus scraper will think it is an error.

After I hit my health check endpoint it caused a counter to be modified which resulted in the metrics export returning content finally:

    <PackageReference Include="OpenTelemetry" Version="1.3.0" />
    <PackageReference Include="OpenTelemetry.Exporter.Console" Version="1.3.0" />
    <PackageReference Include="OpenTelemetry.Exporter.Jaeger" Version="1.3.0" />
    <PackageReference Include="OpenTelemetry.Exporter.Prometheus" Version="1.3.0-rc.2" />
    <PackageReference Include="OpenTelemetry.Extensions.Hosting" Version="1.0.0-rc9.4" />
    <PackageReference Include="OpenTelemetry.Instrumentation.AspNetCore" Version="1.0.0-rc9.4" />
    <PackageReference Include="OpenTelemetry.Instrumentation.Http" Version="1.0.0-rc9.4" />

Microsoft.AspNetCore.App 6.0.5
Microsoft.NETCore.App 6.0.5

[CodeBlanch]

@Yun-Ting You want to tackle this? We looked for something documented about what the status code should be for success-no-data case but couldn't find anything so we went with 204. The code above shows the scraper behavior I think that is valid enough documentation to switch it to 200.

/cc @reyang