Azure App Service RoleName for Serilog

RoleNameContainer.cs

/// <summary>
/// We need to check the App Service Kudo header WAS-DEFAULT-HOSTNAME.
/// The environment variable "WEBSITE_HOSTNAME" will not be updated to production after swapping App Service slots.
/// </summary>
public class RoleNameContainer
{
    // See internal class https://github.com/microsoft/ApplicationInsights-dotnet/blob/2.19.0/NETCORE/src/Microsoft.ApplicationInsights.AspNetCore/Implementation/RoleNameContainer.cs
    // https://github.com/microsoft/ApplicationInsights-dotnet/blob/51dc14afaf7de2b6902a10d6518d2d400cb61f67/NETCORE/src/Microsoft.ApplicationInsights.AspNetCore/DiagnosticListeners/Implementation/HostingDiagnosticListener.cs#L192

    private const string WebAppHostNameHeaderName = "WAS-DEFAULT-HOSTNAME";
    private const string WebAppHostNameEnvironmentVariable = "WEBSITE_HOSTNAME";

    private string roleName = string.Empty;

    /// <summary>
    /// Initializes a new instance of the <see cref="Microsoft.ApplicationInsights.AspNetCore.Implementation.RoleNameContainer"/> class.
    /// Will set the RoleName based on an environment variable.
    /// </summary>
    /// <param name="hostNameSuffix">Host name suffix will be used to parse the prefix from the host name. The value of the prefix is the RoleName.</param>
    internal RoleNameContainer(string hostNameSuffix = ".azurewebsites.net")
    {
        this.HostNameSuffix = hostNameSuffix;
        var enVarValue = Environment.GetEnvironmentVariable(WebAppHostNameEnvironmentVariable);
        this.ParseAndSetRoleName(enVarValue);

        this.IsAzureWebApp = !string.IsNullOrEmpty(enVarValue);
    }

    /// <summary>
    /// Gets or sets static instance for Initializer and DiagnosticListener to share access to RoleName variable.
    /// </summary>
    public static RoleNameContainer Instance { get; set; }

    /// <summary>
    /// Gets or sets role name of the current application.
    /// </summary>
    public string RoleName
    {
        get => this.roleName;

        set
        {
            if (value != this.roleName)
            {
                Interlocked.Exchange(ref this.roleName, value);
            }
        }
    }

    /// <summary>
    /// Gets a value indicating whether indicates if the current app is an Azure Web App based on the presence of a specific environment variable. Set in constructor.
    /// </summary>
    public bool IsAzureWebApp { get; private set; }

    /// <summary>
    /// Gets suffix of website name. This must be changed when running in non public Azure region.
    /// Default value (Public Cloud):  ".azurewebsites.net"
    /// For US Gov Cloud: ".azurewebsites.us"
    /// For Azure Germany: ".azurewebsites.de".
    /// </summary>
    public string HostNameSuffix { get; private set; }

    /// <summary>
    /// Attempt to set the role name from a given collection of request headers.
    /// </summary>
    /// <param name="requestHeaders">Request headers to check for role name.</param>
    public void Set(IHeaderDictionary requestHeaders)
    {
        string headerValue = requestHeaders[WebAppHostNameHeaderName];
        this.ParseAndSetRoleName(headerValue);
    }

    private void ParseAndSetRoleName(string input)
    {
        if (string.IsNullOrEmpty(input))
        {
            // do nothing
        }
        else if (input.EndsWith(this.HostNameSuffix, StringComparison.OrdinalIgnoreCase))
        {
            this.RoleName = input.Substring(0, input.Length - this.HostNameSuffix.Length);
        }
        else
        {
            this.RoleName = input;
        }
    }
}

RoleNameContainerExtensions.cs

public static class RoleNameContainerExtensions
{
    /// <summary>
    /// Gets the role name from App Service Kudo header
    /// </summary>
    public static IApplicationBuilder UseRoleNameContainer(this IApplicationBuilder app)
    {
        app.Use(next => async context =>
        {
            RoleNameContainer.Instance ??= new RoleNameContainer();
            RoleNameContainer.Instance.Set(context.Request.Headers);
            await next(context);
        });
        return app;
    }
}

SerilogAppInsightsTraceTelemetryConverter.cs

public class SerilogAppInsightsTraceTelemetryConverter : TraceTelemetryConverter
{
    // SeriLog does not populate the cloud_RoleName property:
    // https://github.com/serilog/serilog-sinks-applicationinsights/issues/152

    public override IEnumerable<ITelemetry> Convert(LogEvent logEvent, IFormatProvider formatProvider)
    {
        foreach (var telemetry in base.Convert(logEvent, formatProvider))
        {

            if (RoleNameContainer.Instance?.IsAzureWebApp ?? false)
                if (string.IsNullOrEmpty(telemetry.Context.Cloud.RoleName))
                    telemetry.Context.Cloud.RoleName = RoleNameContainer.Instance.RoleName;

            yield return telemetry;
        }
    }
}