httpclientfactory-simple.cs

public void ConfigureServices(IServiceCollection services)
{
    services.AddHttpClient();    
    services.AddSingleton<CosmosClient>(serviceProvider =>
    {
        IHttpClientFactory httpClientFactory = serviceProvider.GetRequiredService<IHttpClientFactory>();

        CosmosClientOptions cosmosClientOptions = new CosmosClientOptions()
        {
            HttpClientFactory = httpClientFactory.CreateClient
        };

        return new CosmosClient("<connection-string>", cosmosClientOptions);
    });
}