如何为AddOutgoingGoogleTraceHandler生成的链路追踪添加自定义标签？

针对v4.3.0版本的Google.Cloud.Diagnostics.AspNetCore库，有两种兼容的方案可以为AddOutgoingGoogleTraceHandler生成的出站请求Span追加自定义标签：

方法1：自定义DelegatingHandler按需追加标签 #

该方案可以绑定HTTP请求/响应的上下文信息灵活加标签，是最常用的实现方式：

• 先定义自定义请求处理器

public class CustomOutgoingTraceTagHandler : DelegatingHandler
{
    private readonly ITraceContextAccessor _traceContextAccessor;

    public CustomOutgoingTraceTagHandler(ITraceContextAccessor traceContextAccessor)
    {
        _traceContextAccessor = traceContextAccessor;
    }

    protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        // 拿到Google追踪处理器生成的当前活跃Span
        if (_traceContextAccessor.TraceContext is ISpanContext currentSpan)
        {
            // 按需添加请求相关自定义标签
            currentSpan.SetLabel("custom_request_domain", request.RequestUri?.Host ?? "unknown");
            currentSpan.SetLabel("custom_request_method", request.Method.Method);
        }

        var response = await base.SendAsync(request, cancellationToken);

        // 也可以拿到响应后追加响应维度的标签
        if (_traceContextAccessor.TraceContext is ISpanContext responseSpan)
        {
            responseSpan.SetLabel("custom_response_status", (int)response.StatusCode);
        }

        return response;
    }
}

• 调整HTTP客户端的处理器注册顺序

// 注册自定义处理器的依赖
builder.Services.AddTransient<CustomOutgoingTraceTagHandler>();

// 注册HTTP客户端时，自定义处理器必须放在AddOutgoingGoogleTraceHandler之后
builder.Services.AddHttpClient("YourTargetHttpClient")
    .AddOutgoingGoogleTraceHandler()
    .AddHttpMessageHandler<CustomOutgoingTraceTagHandler>();

注意：处理器顺序不能颠倒，否则无法拿到对应请求生成的活跃Span

方法2：全局TraceProcessor统一批量加标签 #

如果需要给所有出站请求Span统一加固定规则的标签，不需要绑定单请求上下文，可以用该方案：

• 定义自定义Trace处理器

public class CustomOutgoingTraceProcessor : ITraceProcessor
{
    public void Process(Trace trace)
    {
        // 筛选所有出站HTTP请求的Span，默认生成的Span名称前缀为HttpOutgoing
        foreach (var span in trace.Spans.Where(s => s.Name.StartsWith("HttpOutgoing")))
        {
            // 统一追加全局自定义标签
            span.Labels["custom_service_version"] = "v1.2.3";
            span.Labels["custom_deploy_env"] = "production";
        }
    }
}

• 注册到全局追踪配置中

builder.Services.AddGoogleDiagnosticsForAspNetCore(options =>
{
    options.TraceOptions.TraceProcessors.Add(new CustomOutgoingTraceProcessor());
});

内容的提问来源于stack exchange，提问作者Gregordy