feat: add tracing enrichment and prometheus exporter

src/Core/TakeoutSaaS.Shared.Abstractions/Diagnostics/TraceContext.cs

@@ -3,11 +3,12 @@ using System.Threading;
 namespace TakeoutSaaS.Shared.Abstractions.Diagnostics;
 
 /// <summary>
-/// 轻量级 TraceId 上下文，便于跨层访问当前请求的追踪标识。
+/// 轻量级 TraceId/SpanId 上下文，便于跨层访问当前请求的追踪标识。
 /// </summary>
 public static class TraceContext
 {
     private static readonly AsyncLocal<string?> TraceIdHolder = new();
+    private static readonly AsyncLocal<string?> SpanIdHolder = new();
 
     /// <summary>
     /// 当前请求的 TraceId。
@@ -18,8 +19,21 @@ public static class TraceContext
         set => TraceIdHolder.Value = value;
     }
 
+    /// <summary>
+    /// 当前请求的 SpanId。
+    /// </summary>
+    public static string? SpanId
+    {
+        get => SpanIdHolder.Value;
+        set => SpanIdHolder.Value = value;
+    }
+
     /// <summary>
     /// 清理 TraceId，避免 AsyncLocal 污染其它请求。
     /// </summary>
-    public static void Clear() => TraceIdHolder.Value = null;
+    public static void Clear()
+    {
+        TraceIdHolder.Value = null;
+        SpanIdHolder.Value = null;
+    }
 }