feat(metrics): Add "unknown" for dest labels on local replies

Because `destination_*` tags are generated at the request's destination,
requests that invoke a local response from Envoy have no `destination_*`
values. This change modifies Envoy to add the expected header keys with
the value "unknown" to stay consistent with the behavior for requests
that don't invoke a local reply.

Signed-off-by: Jon Huhn <johuhn@microsoft.com>

pkg/envoy/lds/connection_manager.go

@@ -1,6 +1,8 @@
 package lds
 
 import (
+	envoy_config_accesslog_v3 "github.com/envoyproxy/go-control-plane/envoy/config/accesslog/v3"
+	envoy_config_core_v3 "github.com/envoyproxy/go-control-plane/envoy/config/core/v3"
 	xds_route "github.com/envoyproxy/go-control-plane/envoy/config/route/v3"
 	xds_hcm "github.com/envoyproxy/go-control-plane/envoy/extensions/filters/network/http_connection_manager/v3"
 	"github.com/envoyproxy/go-control-plane/pkg/wellknown"
@@ -77,6 +79,31 @@ func getHTTPConnectionManager(routeName string, cfg configurator.Configurator, h
 			filters = append(filters, statsFilter)
 		}
 		connManager.HttpFilters = append(filters, connManager.HttpFilters...)
+
+		// When Envoy responds to an outgoing HTTP request with a local reply,
+		// destination_* tags for WASM metrics are missing. This configures
+		// Envoy's local replies to add the same headers that are expected from
+		// HTTP responses with the "unknown" value hardcoded because we don't
+		// know the intended destination of the request.
+		var localReplyHeaders []*envoy_config_core_v3.HeaderValueOption
+		for k := range headers {
+			localReplyHeaders = append(localReplyHeaders, &envoy_config_core_v3.HeaderValueOption{
+				Header: &envoy_config_core_v3.HeaderValue{
+					Key:   k,
+					Value: "unknown",
+				},
+			})
+		}
+		connManager.LocalReplyConfig = &xds_hcm.LocalReplyConfig{
+			Mappers: []*xds_hcm.ResponseMapper{
+				{
+					Filter: &envoy_config_accesslog_v3.AccessLogFilter{
+						FilterSpecifier: &envoy_config_accesslog_v3.AccessLogFilter_NotHealthCheckFilter{},
+					},
+					HeadersToAdd: localReplyHeaders,
+				},
+			},
+		}
 	}
 
 	return connManager

pkg/envoy/lds/listener_test.go

@@ -150,6 +150,8 @@ var _ = Describe("Test getHTTPConnectionManager", func() {
 			Expect(connManager.GetHttpFilters()[2].GetName()).To(Equal(wellknown.HTTPRoleBasedAccessControl))
 			Expect(connManager.GetHttpFilters()[3].GetName()).To(Equal(wellknown.Router))
 
+			Expect(connManager.GetLocalReplyConfig().GetMappers()[0].HeadersToAdd[0].Header.Value).To(Equal("unknown"))
+
 			// reset global state
 			statsWASMBytes = oldStatsWASMBytes
 			featureflags.Features.WASMStats = oldWASMflag