site: document support for custom Envoy JSON Fields

follow-up for projectcontour#3059

site/_guides/structured-logs.md

@@ -23,10 +23,17 @@ To enable the feature you have two options:
 
 ## Customizing logged fields
 
-To customize the logged fields, add a `json-fields` list of strings to your config file.
-These strings must be options from the [list of valid fields][1].
-If the `json-fields` key is not specified, the [default fields][2] will be configured.
-Field names not in that list will be silently dropped. (This is not ideal, watch [#1507][3] for updates.)
+To customize the logged fields, add a `json-fields` list of strings to your
+config file.  These strings must be options from the [list of valid fields][1]
+or a valid key/value pair as described below.  If the `json-fields` key is not
+specified, the [default fields][2] will be configured.
+
+To use Envoy supported logging operators that are not part of the list of valid
+fields, or which otherwise require additional customization, they can be
+specified as key/value pairs in the form `fieldName=%OPERATOR(...)%`.
+
+Unknown field names in non key/value fields will result in validation errors, as
+will unknown Envoy operators in key/value fields.
 
 The [example config file][4] contains the full list of fields as well.
 
@@ -41,6 +48,7 @@ json-fields:
   - "authority"
   - "bytes_received"
   - "bytes_sent"
+  - "customer_id=%REQ(X-CUSTOMER-ID)%"
   - "downstream_local_address"
   - "downstream_remote_address"
   - "duration"