not 'a metadata'

doc/xds-test-descriptions.md

@@ -556,7 +556,7 @@ There are four sub-tests:
 
 1. `app_timeout_exceeded`
    1. Test client configured to send UnaryCall RPCs with a 1s application
-      timeout, and a metadata of `rpc-behavior: sleep-2`.
+      timeout, and metadata of `rpc-behavior: sleep-2`.
    1. Test driver asserts client recieves ~100% status `DEADLINE_EXCEEDED`.
 1. `timeout_not_exceeded`
    1. Test client configured to send UnaryCall RPCs with the default
@@ -565,7 +565,7 @@ There are four sub-tests:
 1. `timeout_exceeded` (executed with the below test case)
 1. `timeout_different_route`
    1. Test client configured to send UnaryCall RPCs and EmptyCall RPCs with
-      the default application timeout (20 seconds), and a metadata of
+      the default application timeout (20 seconds), and metadata of
       `rpc-behavior: sleep-4`.
    1. Test driver asserts client recieves ~100% status `OK` for EmptyCall
       and ~100% status `DEADLINE_EXCEEDED` for UnaryCall.