xds: move env var check for HTTP CONNECT metadata parsing to endpoint and locality parsing functions (#8551)

Currently, the env var check for parsing HTTP CONNECT metadata (A86) is
inside the function that parses custom metadata,
`validateAndConstructMetadata`.

This PR moves the check to the endpoint and locality parsing functions,
`parseEndpoint` and the top-level `parseEDSRespProto` which is where
localities are parsed. This allows multiple env vars to control
different custom metadata keys. We already support two custom metadata
keys (A76 and A86) and we plan to support more (A83).

This PR also ensures that the custom metadata used for ring_hash key
(A76) uses the recently added `StructMetadataValue` type. This ensures
that metadata parsing happens only once.

Since the location of the env var check is moved, the tests are also
restructured a little. This PR groups the custom metadata parsing tests
into three groups: one for success cases when the env var is turned on,
one for success cases when the env var is turned off, and one for
failure cases when the env var is turned on.

RELEASE NOTES: none

Author: easwars

internal/xds/xdsclient/xdsresource/unmarshal_eds.go

@@ -108,35 +108,42 @@ func parseEndpoints(lbEndpoints []*v3endpointpb.LbEndpoint, uniqueEndpointAddrs
 			}
 			uniqueEndpointAddrs[a] = true
 		}
-		endpointMetadata, err := validateAndConstructMetadata(lbEndpoint.GetMetadata())
-		if err != nil {
-			return nil, err
+
+		var endpointMetadata map[string]any
+		var hashKey string
+		if envconfig.XDSHTTPConnectEnabled || !envconfig.XDSEndpointHashKeyBackwardCompat {
+			var err error
+			endpointMetadata, err = validateAndConstructMetadata(lbEndpoint.GetMetadata())
+			if err != nil {
+				return nil, err
+			}
+
+			// "The xDS resolver, described in A74, will be changed to set the hash_key
+			// endpoint attribute to the value of LbEndpoint.Metadata envoy.lb hash_key
+			// field, as described in Envoy's documentation for the ring hash load
+			// balancer." - A76
+			if !envconfig.XDSEndpointHashKeyBackwardCompat {
+				hashKey = hashKeyFromMetadata(endpointMetadata)
+			}
 		}
 		endpoints = append(endpoints, Endpoint{
 			HealthStatus: EndpointHealthStatus(lbEndpoint.GetHealthStatus()),
 			Addresses:    addrs,
 			Weight:       weight,
-			HashKey:      hashKey(lbEndpoint),
+			HashKey:      hashKey,
 			Metadata:     endpointMetadata,
 		})
 	}
 	return endpoints, nil
 }
 
-// hashKey extracts and returns the hash key from the given LbEndpoint. If no
-// hash key is found, it returns an empty string.
-func hashKey(lbEndpoint *v3endpointpb.LbEndpoint) string {
-	// "The xDS resolver, described in A74, will be changed to set the hash_key
-	// endpoint attribute to the value of LbEndpoint.Metadata envoy.lb hash_key
-	// field, as described in Envoy's documentation for the ring hash load
-	// balancer." - A76
-	if envconfig.XDSEndpointHashKeyBackwardCompat {
-		return ""
-	}
-	envoyLB := lbEndpoint.GetMetadata().GetFilterMetadata()["envoy.lb"]
-	if envoyLB != nil {
-		if h := envoyLB.GetFields()["hash_key"]; h != nil {
-			return h.GetStringValue()
+// hashKey extracts and returns the hash key from the given endpoint metadata.
+// If no hash key is found, it returns an empty string.
+func hashKeyFromMetadata(metadata map[string]any) string {
+	envoyLB, ok := metadata["envoy.lb"].(StructMetadataValue)
+	if ok {
+		if h, ok := envoyLB.Data["hash_key"].(string); ok {
+			return h
 		}
 	}
 	return ""
@@ -195,9 +202,13 @@ func parseEDSRespProto(m *v3endpointpb.ClusterLoadAssignment) (EndpointsUpdate,
 		if err != nil {
 			return EndpointsUpdate{}, err
 		}
-		localityMetadata, err := validateAndConstructMetadata(locality.GetMetadata())
-		if err != nil {
-			return EndpointsUpdate{}, err
+		var localityMetadata map[string]any
+		if envconfig.XDSHTTPConnectEnabled {
+			var err error
+			localityMetadata, err = validateAndConstructMetadata(locality.GetMetadata())
+			if err != nil {
+				return EndpointsUpdate{}, err
+			}
 		}
 
 		ret.Localities = append(ret.Localities, Locality{
@@ -217,9 +228,7 @@ func parseEDSRespProto(m *v3endpointpb.ClusterLoadAssignment) (EndpointsUpdate,
 }
 
 func validateAndConstructMetadata(metadataProto *v3corepb.Metadata) (map[string]any, error) {
-	// TODO(easwars): Find a better place for the environment variable check
-	// once A83 is implemented.
-	if !envconfig.XDSHTTPConnectEnabled || metadataProto == nil {
+	if metadataProto == nil {
 		return nil, nil
 	}
 	metadata := make(map[string]any)