Add backend routing capture to vMCP audit logs

[yrobla]

Enhance audit logging to capture which backend workload handled each MCP request (tools/call, resources/read, prompts/get). Backend routing information is now recorded in audit events as backend_name in the metadata.extra field.

Partially-closes: #2980

Large PR Justification

This is an atomic PR, including complete tests, that are needed to properly validate the feature.

[codecov]

Codecov Report

❌ Patch coverage is 96.29630% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.25%. Comparing base (61a84b3) to head (4ef10a4).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/audit/auditor.go	84.61%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@ Coverage Diff @@ ## main #2983 +/- ## ========================================== + Coverage 56.18% 56.25% +0.07%  ========================================== Files 330 331 +1 Lines 32468 32520 +52 ========================================== + Hits 18243 18295 +52  + Misses 12676 12675 -1  - Partials 1549 1550 +1 

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[Copilot]

Pull request overview

This PR enhances vMCP audit logging to capture backend routing information, recording which backend workload handled each MCP request (tools/call, resources/read, prompts/get) in the audit event metadata. The implementation uses a mutable context-based approach where the audit middleware creates a BackendInfo struct, and a new backend enrichment middleware populates it by looking up the routing table.

Key Changes

• Added backend enrichment middleware that parses MCP requests and looks up backend names from the routing table
• Extended audit event metadata to include backend_name when available
• Introduced BackendInfo context helper to enable cross-middleware data sharing

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
pkg/vmcp/server/server.go	Registers backend enrichment middleware in the handler chain between discovery and telemetry
pkg/vmcp/server/backend_enrichment.go	New middleware that extracts backend routing information from MCP requests and populates BackendInfo in context
pkg/audit/auditor.go	Adds BackendInfo context helpers and updates addMetadata to include backend_name in audit events
pkg/audit/backend_info_test.go	Unit tests for BackendInfo context storage and mutation behavior
pkg/audit/auditor_test.go	Updates TestAddMetadata to accommodate new request parameter in addMetadata signature
pkg/vmcp/server/integration_test.go	Extends audit logging test to verify backend_name is captured in tools/call and resources/read audit events

Comments suppressed due to low confidence (1)

pkg/audit/auditor_test.go:528

• The test for addMetadata should verify that backend_name is only added to metadata.extra when BackendInfo is present in the request context with a non-empty BackendName. Consider adding test cases to verify: (1) backend_name is added when BackendInfo exists with a name, (2) backend_name is not added when BackendInfo is missing, and (3) backend_name is not added when BackendInfo.BackendName is empty.

func TestAddMetadata(t *testing.T) {	t.Parallel()	auditor, err := NewAuditorWithTransport(&Config{}, "sse")	require.NoError(t, err)	event := NewAuditEvent("test", EventSource{}, OutcomeSuccess, map[string]string{}, "test")	duration := 150 * time.Millisecond	rw := &responseWriter{	ResponseWriter: httptest.NewRecorder(),	body: bytes.NewBufferString("test response"),	}	req := httptest.NewRequest("GET", "/test", nil)	auditor.addMetadata(event, req, duration, rw)	require.NotNil(t, event.Metadata.Extra)	assert.Equal(t, int64(150), event.Metadata.Extra[MetadataExtraKeyDuration])	assert.Equal(t, "sse", event.Metadata.Extra[MetadataExtraKeyTransport])	assert.Equal(t, 13, event.Metadata.Extra[MetadataExtraKeyResponseSize]) // "test response" length } 

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[JAORMX]

Yay! This is exactly how to use the audit log's metada 😀

[github-actions]

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[github-actions]

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