Monitoring C# Azure Functions in the Isolated Worker Model - Infrastructure & Configuration Deep Dive

Not so long ago my colleague reached out with a question "Are there any known issues with ITelemetryInitializer in Azure Functions?". This question started a discussion about the monitoring configuration for C# Azure Functions in the isolated worker model. At some point in that discussion I stated "Alright, you've motivated me, I'll make a sample". When I sat down to do that, I started wondering which scenario should I cover and my conclusion was that there are several things I should go through... So here I am.

Setting the Scene

Before I start describing how we can monitor an isolated worker model C# Azure Function, allow me to introduce you to the one we are going to use for this purpose. I want to start with as basic setup as possible. This is why the initial Program.cs will contain only four lines.

var host = new HostBuilder()
    .ConfigureFunctionsWebApplication()
    .Build();

host.Run();

The host.json will also be minimal.

{
    "version": "2.0",
    "logging": {
        "logLevel": {
            "default": "Information"
        }
    }
}

For the function itself, I've decided to go for the Fibonacci sequence implementation as it can easily generate a ton of logs.

public class FibonacciSequence
{
    private readonly ILogger<FibonacciSequence> _logger;

    public FibonacciSequence(ILogger<FibonacciSequence> logger)
    {
        _logger = logger;
    }

    [Function(nameof(FibonacciSequence))]
    public async Task<HttpResponseData> Run(
        [HttpTrigger(AuthorizationLevel.Anonymous, "get", Route = "fib/{index:int}")]
        HttpRequestData request,
        int index)
    {
        _logger.LogInformation(
            $"{nameof(FibonacciSequence)} function triggered for {{index}}.",
            index
        );

        var response = request.CreateResponse(HttpStatusCode.OK)