[BUG] [Monitor] Logs sent through AzureMonitorLogExporter are not interpolated properly

[mareklinka]

Library name and version

Azure.Monitor.OpenTelemetry.AspNetCore 1.2.0

Describe the bug

Hello all. I have an ASP.NET 8 application which is currently using the old Application Insights SDK to ship logs and metrics to Application Insights. I am currently in the process of replacing the old SDK with the new OpenTelemetry package.

Our application uses Serilog for logging, which has a dedicated Application Insights sink. Since there is no official Serilog sink using the OTel package, I had to write my own to get feature parity. I implemented the sink according to the migration docs. However, when logging messages to the OTel-enabled ILogger, the messages arrive into Application Insights without being interpolated.

Expected behavior

Log messages are properly interpolated so that they are easily readable. Instead of Message with placeholder: {hole}, the message should be Message with placeholder: 42, same as what the console logger does:

Actual behavior

Logs show up in Application Insights without interpolation, with correct customDimensions:

Reproduction Steps

I created a small repro using a console application:

using Azure.Monitor.OpenTelemetry.Exporter;
using Microsoft.Extensions.Logging;
using OpenTelemetry.Resources;

using var loggerFactory = LoggerFactory.Create(builder => builder
    .AddConsole()
    .AddOpenTelemetry(logging =>
    {
            logging.SetResourceBuilder(ResourceBuilder.CreateDefault());
            logging.AddAzureMonitorLogExporter(c => c.ConnectionString = "...");
    }));

var logger = loggerFactory.CreateLogger<Program>();

logger.LogInformation("Message with placeholder: {hole}", 42);

Console.ReadLine();

Environment

VS Code 1.95.1 + C# + C# DevKit.

.NET SDK:
 Version:           9.0.100-rc.2.24474.11
 Commit:            315e1305db
 Workload version:  9.0.100-manifests.cea95dba
 MSBuild version:   17.12.0-preview-24473-03+fea15fbd1

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.22631
 OS Platform: Windows
 RID:         win-x64
 Base Path:   C:\Program Files\dotnet\sdk\9.0.100-rc.2.24474.11\

.NET workloads installed:
 [aspire]
   Installation Source: VS 17.11.35327.3
   Manifest Version:    8.2.0/8.0.100
   Manifest Path:       C:\Program Files\dotnet\sdk-manifests\8.0.100\microsoft.net.sdk.aspire\8.2.0\WorkloadManifest.json
   Install Type:              Msi

Configured to use loose manifests when installing new manifests.

Host:
  Version:      9.0.0-rc.2.24473.5
  Architecture: x64
  Commit:       990ebf52fc

.NET SDKs installed:
  8.0.403 [C:\Program Files\dotnet\sdk]
  9.0.100-rc.2.24474.11 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.App 8.0.10 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 9.0.0-rc.2.24474.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 6.0.35 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 8.0.10 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 9.0.0-rc.2.24473.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 6.0.35 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 8.0.10 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 9.0.0-rc.2.24474.4 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Other architectures found:
  x86   [C:\Program Files (x86)\dotnet]
    registered at [HKLM\SOFTWARE\dotnet\Setup\InstalledVersions\x86\InstallLocation]

Environment variables:
  Not set

global.json file:
  Not found

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  https://aka.ms/dotnet/download

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