OpenAI: Add Suffix to v1/completions (#5611)

* add suffix

* remove todo

* remove TODO

* add to test

* rm outdated prompt tokens info md

* fix test

* fix test

openai/openai_test.go

@@ -85,6 +85,7 @@ func TestMiddlewareRequests(t *testing.T) {
 					Prompt:      "Hello",
 					Temperature: &temp,
 					Stop:        []string{"\n", "stop"},
+					Suffix:      "suffix",
 				}
 
 				bodyBytes, _ := json.Marshal(body)
@@ -115,6 +116,10 @@ func TestMiddlewareRequests(t *testing.T) {
 				if stopTokens[0] != "\n" || stopTokens[1] != "stop" {
 					t.Fatalf("expected ['\\n', 'stop'], got %v", stopTokens)
 				}
+
+				if genReq.Suffix != "suffix" {
+					t.Fatalf("expected 'suffix', got %s", genReq.Suffix)
+				}
 			},
 		},
 		{