Update ingress gateway stats labeling

In the absence of stats_tags to handle this pattern, when we pass
"ingress_upstream.$port" as the stat_prefix, Envoy splits up that prefix
and makes the port a part of the metric name.

For example:
- stat_prefix: ingress_upstream.8080

This leads to metric names like envoy_http_8080_no_route. Changing the
stat_prefix to ingress_upstream_80880 yields the expected metric names
such as envoy_http_no_route.

Note that we don't encode the destination's name/ns/dc in this
stat_prefix because for HTTP services ingress gateways use a single
filter chain. Only cluster metrics are available on a per-upstream
basis.

agent/xds/listeners.go

@@ -536,7 +536,7 @@ func (s *ResourceGenerator) makeIngressGatewayListeners(address string, cfgSnap
 				filterName:      listenerKey.RouteName(),
 				routeName:       listenerKey.RouteName(),
 				cluster:         "",
-				statPrefix:      "ingress_upstream.",
+				statPrefix:      "ingress_upstream_",
 				routePath:       "",
 				httpAuthzFilter: nil,
 			}

agent/xds/testdata/listeners/ingress-http-multiple-services.envoy-1-18-x.golden

@@ -17,7 +17,7 @@
               "name": "envoy.filters.network.http_connection_manager",
               "typedConfig": {
                 "@type": "type.googleapis.com/envoy.extensions.filters.network.http_connection_manager.v3.HttpConnectionManager",
-                "statPrefix": "ingress_upstream.443",
+                "statPrefix": "ingress_upstream_443",
                 "rds": {
                   "configSource": {
                     "ads": {
@@ -60,7 +60,7 @@
               "name": "envoy.filters.network.http_connection_manager",
               "typedConfig": {
                 "@type": "type.googleapis.com/envoy.extensions.filters.network.http_connection_manager.v3.HttpConnectionManager",
-                "statPrefix": "ingress_upstream.8080",
+                "statPrefix": "ingress_upstream_8080",
                 "rds": {
                   "configSource": {
                     "ads": {

agent/xds/testdata/listeners/ingress-http-multiple-services.v2compat.envoy-1-16-x.golden

@@ -17,7 +17,7 @@
               "name": "envoy.filters.network.http_connection_manager",
               "typedConfig": {
                 "@type": "type.googleapis.com/envoy.config.filter.network.http_connection_manager.v2.HttpConnectionManager",
-                "statPrefix": "ingress_upstream.443",
+                "statPrefix": "ingress_upstream_443",
                 "rds": {
                   "configSource": {
                     "ads": {
@@ -60,7 +60,7 @@
               "name": "envoy.filters.network.http_connection_manager",
               "typedConfig": {
                 "@type": "type.googleapis.com/envoy.config.filter.network.http_connection_manager.v2.HttpConnectionManager",
-                "statPrefix": "ingress_upstream.8080",
+                "statPrefix": "ingress_upstream_8080",
                 "rds": {
                   "configSource": {
                     "ads": {

agent/xds/testdata/listeners/ingress-splitter-with-resolver-redirect.envoy-1-18-x.golden

@@ -17,7 +17,7 @@
               "name": "envoy.filters.network.http_connection_manager",
               "typedConfig": {
                 "@type": "type.googleapis.com/envoy.extensions.filters.network.http_connection_manager.v3.HttpConnectionManager",
-                "statPrefix": "ingress_upstream.9191",
+                "statPrefix": "ingress_upstream_9191",
                 "rds": {
                   "configSource": {
                     "ads": {

agent/xds/testdata/listeners/ingress-splitter-with-resolver-redirect.v2compat.envoy-1-16-x.golden

@@ -17,7 +17,7 @@
               "name": "envoy.filters.network.http_connection_manager",
               "typedConfig": {
                 "@type": "type.googleapis.com/envoy.config.filter.network.http_connection_manager.v2.HttpConnectionManager",
-                "statPrefix": "ingress_upstream.9191",
+                "statPrefix": "ingress_upstream_9191",
                 "rds": {
                   "configSource": {
                     "ads": {