feat(contrib/mark3labs/mcp-go): set session ID on tool spans at start time

Pass llmobs.WithSessionID at StartToolSpan so LLMObs groups tool spans under
their MCP session natively (in addition to the existing mcp_session_id tag).
The session is read from the MCP client session attached to ctx.

Inspired by changes in the internal dd-source copy of this contrib.

Author: jboolean

contrib/mark3labs/mcp-go/tracing.go

@@ -33,7 +33,13 @@ func appendTracingHooks(hooks *server.Hooks) {
 
 var toolHandlerMiddleware = func(next server.ToolHandlerFunc) server.ToolHandlerFunc {
 	return func(ctx context.Context, request mcp.CallToolRequest) (*mcp.CallToolResult, error) {
-		toolSpan, ctx := llmobs.StartToolSpan(ctx, request.Params.Name, llmobs.WithIntegration(string(instrumentation.PackageMark3LabsMCPGo)))
+		startOpts := []llmobs.StartSpanOption{
+			llmobs.WithIntegration(string(instrumentation.PackageMark3LabsMCPGo)),
+		}
+		if session := server.ClientSessionFromContext(ctx); session != nil {
+			startOpts = append(startOpts, llmobs.WithSessionID(session.SessionID()))
+		}
+		toolSpan, ctx := llmobs.StartToolSpan(ctx, request.Params.Name, startOpts...)
 
 		var result *mcp.CallToolResult
 		var err error

contrib/mark3labs/mcp-go/tracing_test.go

@@ -178,6 +178,9 @@ func TestIntegrationToolCallSuccess(t *testing.T) {
 	assert.Contains(t, initSpan.Tags, expectedTag)
 	assert.Contains(t, toolSpan.Tags, expectedTag)
 
+	// Tool span carries the session ID natively so LLMObs groups it under the session.
+	assert.Equal(t, sessionID, toolSpan.SessionID)
+
 	assert.Contains(t, toolSpan.Tags, "mcp_method:tools/call")
 	assert.Contains(t, toolSpan.Tags, "mcp_tool_kind:server")
 	assert.Contains(t, toolSpan.Tags, "mcp_tool:calculator")