in src/Google.Cloud.Functions.Hosting/HostingInternals.cs [140:147]
internal static Type GetFunctionTarget(IConfiguration configuration, Assembly assembly)
{
var options = FunctionsFrameworkOptions.FromConfiguration(configuration);
string? target = options.FunctionTarget;
return target is null
? FindDefaultFunctionType(assembly.GetTypes())
: assembly.GetType(target) ?? throw new InvalidOperationException($"Can't load specified function type '{target}'");
}