add metadata section to docs (#4525)

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

plugin/forward/README.md

@@ -101,6 +101,13 @@ On each endpoint, the timeouts for communication are set as follows:
 * The dial timeout by default is 30s, and can decrease automatically down to 100ms based on early results.
 * The read timeout is static at 2s.
 
+## Metadata
+
+The forward plugin will publish the following metadata, if the *metadata*
+plugin is also enabled:
+
+* `forward/upstream`: the upstream used to forward the request
+
 ## Metrics
 
 If monitoring is enabled (via the *prometheus* plugin) then the following metric are exported: