Add custom LB xDS test description (#32686)

doc/xds-test-descriptions.md

@@ -780,3 +780,55 @@ backends and all requests end with the `OK` status.
 3.  The test driver removes the client configuration to send metadata. The
 driver asserts that during some 10-second interval, traffic is equally
 distributed among the five backends, and all requests end with the `OK` status.
+
+### custom_lb
+This test verifies that a custom load balancer policy can be configured in the
+client. It also verifies that when given a list of policies the client can
+ignore a bad one and try the next one on the list until it finds a good one.
+
+Client parameters:
+
+1.  --num_channels=1
+2.  --qps=100
+
+Load balancer configuration:
+
+One MIG with a single backend.
+
+The `backendService` will have the following `localityLbPolicies` entry:
+```json
+[ 
+  {
+    "customPolicy": {
+      "name": "test.ThisLoadBalancerDoesNotExist",
+      "data": "{ \"foo\": \"bar\" }"
+    }
+  },
+  {
+    "customPolicy": {
+      "name": "test.RpcBehaviorLoadBalancer",
+      "data": "{ \"rpcBehavior\": \"error-code-15\" }"
+    }
+  }
+]
+```
+
+The client **should not** implement the `test.ThisLoadBalancerDoesNotExist`, but
+it **should** implement `test.RpcBehaviorLoadBalancer`. The
+`RpcBehaviorLoadBalancer` implementation should set the rpcBehavior request
+header based on the configuration it is provided. The `rpcBehavior` field value
+in the config should be used as the header value.
+
+Assert:
+
+1. The first custom policy is ignored as the client does not have an
+implementation for it.
+2. The second policy, that **is** implemented by the client, has been applied
+by the client. This can be asserted by confirming that each request has
+failed with the configured error code 15 (DATA_LOSS). We should get this error
+because the test server knows to look for the `rpcBehavior` header and fail
+a request with a provided error code.
+
+Note that while this test is for load balancing, we can get by with a single
+backend as our test load balancer does not perform any actual load balancing,
+instead only applying the `rpcBehavior` header to each request.