docs(zipkin) add default_service_name configuration parameter to do…

…cs (#1451)

The default service name can be used to override the
unknown-service-name spans.

app/_hub/kong-inc/zipkin/index.md

@@ -60,6 +60,10 @@ params:
       description: |
         How often to sample requests that do not contain trace ids.
         Set to `0` to turn sampling off, or to `1` to sample **all** requests.
+    - name: default_service_name
+      required: false
+      description: |
+        The default service name to override the unknown-service-name spans.
     - name: include_credential
       required: true
       default: true