Histogram<decimal> does not report values

[joegoldman2]

Area

area:api

Package Version

Package Name	Version
OpenTelemetry.Api	1.8.1
OpenTelemetry	1.8.1

Runtime Version

net8.0

Description

When using a Histogram<decimal>, values are not reported.

Based on the .NET documentation, Histogram<T> should work with decimal.

Steps to Reproduce

using System.Diagnostics.Metrics;
using OpenTelemetry;
using OpenTelemetry.Metrics;

using var meterProvider = Sdk.CreateMeterProviderBuilder()
    .AddMeter("TestMeter")
    .AddConsoleExporter()
    .Build();

var meter = new Meter("TestMeter");

var histogram = meter.CreateHistogram<decimal>("test-histogram");
histogram.Record(10);

This produces the following output:

Resource associated with Metric:
    telemetry.sdk.name: opentelemetry
    telemetry.sdk.language: dotnet
    telemetry.sdk.version: 1.8.1
    service.name: unknown_service:ConsoleApp1

But when using a Histogram<double> for example, the output is:

Resource associated with Metric:
    telemetry.sdk.name: opentelemetry
    telemetry.sdk.language: dotnet
    telemetry.sdk.version: 1.8.1
    service.name: unknown_service:ConsoleApp1

Metric Name: test-histogram, Meter: TestMeter
(2024-05-14T16:27:14.4076497Z, 2024-05-14T16:27:14.4159669Z] Histogram
Value: Sum: 10 Count: 1 Min: 10 Max: 10
(-Infinity,0]:0
(0,5]:0
(5,10]:1
(10,25]:0
(25,50]:0
(50,75]:0
(75,100]:0
(100,250]:0
(250,500]:0
(500,750]:0
(750,1000]:0
(1000,2500]:0
(2500,5000]:0
(5000,7500]:0
(7500,10000]:0
(10000,+Infinity]:0

Expected Result

Values should be reported when using Histogram<decimal>.

Actual Result

Values are not reported when using Histogram<decimal>.

Additional Context

No response

[joegoldman2]

It's working when using a MeterListener, so I don't think it's a issue with .NET:

var meter = new Meter("TestMeter");
var histogram = meter.CreateHistogram<decimal>("test-histogram");

using var listener = new MeterListener();
listener.EnableMeasurementEvents(histogram);
listener.SetMeasurementEventCallback<decimal>((instrument, measurement, tags, state) =>
{
    Console.WriteLine(measurement);
});

histogram.Record(10);

This produces the following output:

10

[joegoldman2]

I think the issue is here:

opentelemetry-dotnet/src/OpenTelemetry/Metrics/MeterProviderSdk.cs

Lines 173 to 181 in 5aa077b

	// Everything double
	this.listener.SetMeasurementEventCallback<double>(MeasurementRecordedDouble);
	this.listener.SetMeasurementEventCallback<float>(static (instrument, value, tags, state) => MeasurementRecordedDouble(instrument, value, tags, state));
	// Everything long
	this.listener.SetMeasurementEventCallback<long>(MeasurementRecordedLong);
	this.listener.SetMeasurementEventCallback<int>(static (instrument, value, tags, state) => MeasurementRecordedLong(instrument, value, tags, state));
	this.listener.SetMeasurementEventCallback<short>(static (instrument, value, tags, state) => MeasurementRecordedLong(instrument, value, tags, state));
	this.listener.SetMeasurementEventCallback<byte>(static (instrument, value, tags, state) => MeasurementRecordedLong(instrument, value, tags, state));

where SetMeasurementEventCallback<decimal> is missing. And this issue is not specific to Histogram<T> but all instruments that use decimal.

The fix can be a bit complicated (or deep let's say) as MetricPoint itself doesn't support decimal 😞.

[cijothomas]

It is intentional to only support what can be reasonably represented/transported in OTLP format, which do not support decimal, without precision loss.

[joegoldman2]

I understand and yes I had to cast to double (especially in the OpenTelemetry exporter) which means a loss of precision. For financial or scientific applications it's quite common and recommended to use decimal. Is the recommended solution to keep decimal but use metrics that use double in this case?

[cijothomas]

Is the recommended solution

I am sorry, I do not have a recommendation for this yet. I was just pointing out that it was intentional to keep the supported types limited (and remove the bug tag!)

If we are anyway going to lose precision in export side, is it feasible to do the decimal->double by the user when calling the API itself?

[joegoldman2]

I think it could be something feasible and could be the solution for now. As OTLP doesn't support more precision for the moment, I let you decide if you want to close this issue and the associated PR. Thank you for your feedback!

[cijothomas]

I'd close the PR, but keep this issue open, to see if this is a common ask. Fixing this would be likely involve working with OpenTelemetry spec/proto repo, but first step is to gauge how common is this ask.
Thanks for your time.