Prometheus exporter writes boolean values as "True" and "False"

[JamesNK]

Bug Report

List of all OpenTelemetry NuGet
packages and version that you are
using (e.g. OpenTelemetry 1.0.2):

<PackageReference Include="OpenTelemetry.Exporter.Prometheus.AspNetCore" Version="1.6.0-rc.1" />
<PackageReference Include="OpenTelemetry.Extensions.Hosting" Version="1.6.0-rc.1" />

Runtime version (e.g. net462, net48, netcoreapp3.1, net6.0 etc. You can
find this information from the *.csproj file):

net7.0

Symptom

Prometheus exporter writes boolean values as "True" and "False":

What is the expected behavior?

I see the go exporter writes lower case values, e.g. "true" at https://github.com/open-telemetry/opentelemetry-go/blob/02616a25c68e674a04df01a1b7ba185a2db8cd5a/exporters/prometheus/testdata/counter.txt#L3 that is generated from https://github.com/open-telemetry/opentelemetry-go/blob/02616a25c68e674a04df01a1b7ba185a2db8cd5a/exporters/prometheus/exporter_test.go#L51. That seems like a more idiomatic behavior.

What is the actual behavior?

The written value appears to be bool.TrueString and bool.FalseString. Are these the right values?

Reproduce

Create a self-contained project using the template of your choice, apply the
minimum required code to result in the issue you're observing.

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]

The written value appears to be bool.TrueString and bool.FalseString. Are these the right values?

1. They should be "true" and "false".
2. Note that the spec is Experimental and could have breaking changes.

More info:

• The Experimental Prometheus and OpenMetrics Compatibility specification says: "non-string Attribute values MUST be converted to string attributes following the attribute specification"
• The Stable specification says "For protocols that do not natively support non-string values, non-string values SHOULD be represented as JSON-encoded strings"