[BUG] UseAzureMonitor does not appear to support Structured Logging for messages.

[Bpflugrad]

Library name and version

Azure.Monitor.OpenTelemetry.AspNetCore 1.2.0

Describe the bug

When using ILogger for logging with UseAzureMonitor as the OpenTelemetry exporter, Structured Logging is not being performed.

Message arguments are logged as Custom Properties in Application Insights, but the "OriginalFormat" property is not present. The "Message" property contains the interpolated string.

The "OriginalFormat" property is logged for Exceptions, but not messages.

Expected behavior

Traces in Application Insights should include the "OriginalFormat" property, to allow aggregating log messages by their pre-formatted values.

Actual behavior

"OriginalFormat" is not present on (Application Insights) Traces, and is only present for Exceptions.

Reproduction Steps

I've created a minimal example project showing the behaviors.

Example controller:

using Microsoft.AspNetCore.Mvc;

namespace AspireLoggingExample.Api.Controllers;

[ApiController]
[Route("[controller]")]
public class AttributeLogController(ILogger<AttributeLogController> logger) : ControllerBase
{
    [HttpGet(Name = "GetAttributeLog")]

    public string Get()
    {
        const string logMessage = "LogFromAttributeMessage({value})";

        logger.LogFromAttributeMessage(9001);

        return logMessage;
    }
}

internal static partial class LoggerExtensions
{
    [LoggerMessage(LogLevel.Information, "LogFromAttributeMessage({value})")]
    internal static partial void LogFromAttributeMessage(this ILogger logger, int value);
}

Environment

Aspire version: 8.0.1+a6e341ebbf956bbcec0dda304109815fcbae70c9
Microsoft Visual Studio Enterprise 2022 (64-bit): 17.10.3

[github-actions]

Thank you for your feedback. Tagging and routing to the team member best able to assist.

[andrew-lis]

We have a similar problem.

When built in ILogger is used with structured logging that uses "@", the class is not properly logged.

E.g. when log looks like this:

_logger.LogInformation("Test message {@val}");

Then on app insights I can see "Test message { Inner = System.Collections.Generic.Dictionary`2[System.Int32....."
and customdimensions contain the element "@Val" with value as a string, the same as above, not structured log.

What I would expect:

• the message contains a nicely formatted string, e.g. json without .net type names
• customdimensions should contain all elements are from the log as nested objects, not text
• it would be nice to also have "MessageTemplate" field with the template that was used, e.g. "Test message {@Val}". It would help in filtering logs.

[GeorgeF0]

I'm not sure if this is relevant:

azure-sdk-for-net/sdk/monitor/Azure.Monitor.OpenTelemetry.Exporter/src/Internals/LogsHelper.cs

Lines 99 to 109 in 7a804c5

	if (item.Key == "{OriginalFormat}")
	{
	if (logRecord.Exception?.Message != null)
	{
	properties.Add("OriginalFormat", item.Value.ToString().Truncate(SchemaConstants.MessageData_Properties_MaxValueLength)!);
	}
	else if (message == null)
	{
	message = item.Value.ToString();
	}
	}

It seems that OriginalFormat is skipped if it's not an exception. My understanding is that

• if OpenTelemetryLoggerOptions.IncludeFormattedMessage is false, you're supposed to get the unformatted (original) message in the logs (which you do)
• if you have the property set to true (which is set by default by UseAzureMonitor), you're supposed to get both the original as well as the formatted message; which does not seem to be the case.

As @Bpflugrad said, in the latter case you get only the formatted message but not the original (unformatted) one.

Is this the desired behavior? What do we need to do to get both the formatted and original messages?