docs: document UPSTREAM_METADATA header variable (#335)

Signed-off-by: Stephan Zuercher <stephan@turbinelabs.io>

docs/root/configuration/http_conn_man/headers.rst

@@ -289,7 +289,7 @@ documentation.
 **Note:** Headers are appended to requests/responses in the following order: route level headers, virtual host
 level headers and finally global level headers.
 
-Envoy additionally supports adding dynamic values to **request** and **response** headers.
+Envoy additionally supports adding dynamic values to request and response headers.
 Supported dynamic values are:
 
 %CLIENT_IP%
@@ -299,3 +299,12 @@ Supported dynamic values are:
 %PROTOCOL%
     The original protocol which is already added by Envoy as a
     :ref:`x-forwarded-proto <config_http_conn_man_headers_x-forwarded-proto>` request header.
+
+%UPSTREAM_METADATA(["namespace", "key", ...])%
+    Populates the header with ref:`EDS endpoint metadata <envoy_api_file_api/eds.proto>` from the
+    upstream host selected by the router. Metadata may be selected from any namespace. In general,
+    metadata values may be strings, numbers, booleans, lists, nested structures, or null. Upstream
+    metadata values may be selected from nested structs by specifying multiple keys. Otherwise,
+    only string, boolean, and numeric values are supported. If the namespace or key(s) are not
+    found, or if the selected value is not a supported type, then no header is emitted. The
+    namespace and key(s) are specified as a JSON array of strings.