router: add dynamic metadata header formatter

docs/root/configuration/http/http_conn_man/headers.rst

@@ -621,6 +621,12 @@ Supported variable names are:
     Upstream metadata cannot be added to request headers as the upstream host has not been selected
     when custom request headers are generated.
 
+%DYNAMIC_METADATA(["namespace", "key", ...])%
+    Similar to UPSTREAM_METADATA, populates the header with dynamic metadata available in a request
+    (e.g.: added by filters like the header-to-metadata filter).
+
+    This works both on request and response headers.
+
 %UPSTREAM_REMOTE_ADDRESS%
     Remote address of the upstream host. If the address is an IP address it includes both address
     and port. The upstream remote address cannot be added to request headers as the upstream host
@@ -657,4 +663,4 @@ Supported variable names are:
 
 %RESPONSE_CODE_DETAILS%
     Response code details provides additional information about the HTTP response code, such as
-    who set it (the upstream or envoy) and why.
+    who set it (the upstream or envoy) and why.

docs/root/version_history/current.rst

@@ -66,6 +66,7 @@ New Features
   retry policy, which allows retrying envoy's own rate limited responses.
 * router: added new :ref:`host_rewrite_path_regex <envoy_v3_api_field_config.route.v3.RouteAction.host_rewrite_path_regex>`
   option, which allows rewriting Host header based on path.
+* router: added support for DYNAMIC_METADATA :ref:`header formatter <config_http_conn_man_headers_custom_request_headers>`.
 * signal: added support for calling fatal error handlers without envoy's signal handler, via FatalErrorHandler::callFatalErrorHandlers().
 * stats: added optional histograms to :ref:`cluster stats <config_cluster_manager_cluster_stats_request_response_sizes>`
   that track headers and body sizes of requests and responses.

source/common/router/header_formatter.cc

@@ -48,7 +48,7 @@ std::string formatPerRequestStateParseException(absl::string_view params) {
 //   (["a", "b", "c"])
 // There must be at least 2 array elements (a metadata namespace and at least 1 key).
 std::function<std::string(const Envoy::StreamInfo::StreamInfo&)>
-parseUpstreamMetadataField(absl::string_view params_str) {
+parseMetadataField(absl::string_view params_str, bool upstream = true) {
   params_str = StringUtil::trim(params_str);
   if (params_str.empty() || params_str.front() != '(' || params_str.back() != ')') {
     throw EnvoyException(formatUpstreamMetadataParseException(params_str));
@@ -72,14 +72,20 @@ parseUpstreamMetadataField(absl::string_view params_str) {
     throw EnvoyException(formatUpstreamMetadataParseException(params_str));
   }
 
-  return [params](const Envoy::StreamInfo::StreamInfo& stream_info) -> std::string {
-    Upstream::HostDescriptionConstSharedPtr host = stream_info.upstreamHost();
-    if (!host) {
-      return std::string();
+  return [upstream, params](const Envoy::StreamInfo::StreamInfo& stream_info) -> std::string {
+    const envoy::config::core::v3::Metadata* metadata = nullptr;
+    if (upstream) {
+      Upstream::HostDescriptionConstSharedPtr host = stream_info.upstreamHost();
+      if (!host) {
+        return std::string();
+      }
+      metadata = host->metadata().get();
+    } else {
+      metadata = &(stream_info.dynamicMetadata());
     }
 
     const ProtobufWkt::Value* value =
-        &::Envoy::Config::Metadata::metadataValue(host->metadata().get(), params[0], params[1]);
+        &::Envoy::Config::Metadata::metadataValue(metadata, params[0], params[1]);
     if (value->kind_case() == ProtobufWkt::Value::KIND_NOT_SET) {
       // No kind indicates default ProtobufWkt::Value which means namespace or key not
       // found.
@@ -339,8 +345,10 @@ StreamInfoHeaderFormatter::StreamInfoHeaderFormatter(absl::string_view field_nam
       return formatted;
     };
   } else if (absl::StartsWith(field_name, "UPSTREAM_METADATA")) {
+    field_extractor_ = parseMetadataField(field_name.substr(STATIC_STRLEN("UPSTREAM_METADATA")));
+  } else if (absl::StartsWith(field_name, "DYNAMIC_METADATA")) {
     field_extractor_ =
-        parseUpstreamMetadataField(field_name.substr(STATIC_STRLEN("UPSTREAM_METADATA")));
+        parseMetadataField(field_name.substr(STATIC_STRLEN("DYNAMIC_METADATA")), false);
   } else if (absl::StartsWith(field_name, "PER_REQUEST_STATE")) {
     field_extractor_ =
         parsePerRequestStateField(field_name.substr(STATIC_STRLEN("PER_REQUEST_STATE")));

test/common/router/header_formatter_test.cc

@@ -615,6 +615,21 @@ TEST_F(StreamInfoHeaderFormatterTest, TestFormatWithUpstreamMetadataVariableMiss
   testFormatting(stream_info, "UPSTREAM_METADATA([\"namespace\", \"key\"])", "");
 }
 
+TEST_F(StreamInfoHeaderFormatterTest, TestFormatWithRequestMetadata) {
+  NiceMock<Envoy::StreamInfo::MockStreamInfo> stream_info;
+  envoy::config::core::v3::Metadata metadata;
+  ProtobufWkt::Struct struct_obj;
+
+  auto& fields_map = *struct_obj.mutable_fields();
+  fields_map["foo"] = ValueUtil::stringValue("bar");
+  (*metadata.mutable_filter_metadata())["envoy.lb"] = struct_obj;
+
+  EXPECT_CALL(stream_info, dynamicMetadata()).WillRepeatedly(ReturnRef(metadata));
+  EXPECT_CALL(Const(stream_info), dynamicMetadata()).WillRepeatedly(ReturnRef(metadata));
+
+  testFormatting(stream_info, "DYNAMIC_METADATA([\"envoy.lb\", \"foo\"])", "bar");
+}
+
 TEST_F(StreamInfoHeaderFormatterTest, TestFormatWithPerRequestStateVariable) {
   Envoy::StreamInfo::FilterStateSharedPtr filter_state(
       std::make_shared<Envoy::StreamInfo::FilterStateImpl>(