Add successful request count to OpenAI callback (#2128)

I've found it useful to track the number of successful requests to
OpenAI. This gives me a better sense of the efficiency of my prompts and
helps compare map_reduce/refine on a cheaper model vs. stuffing on a
more expensive model with higher capacity.

docs/modules/models/llms/examples/token_usage_tracking.ipynb

@@ -45,7 +45,8 @@
      "text": [
       "Total Tokens: 42\n",
       "Prompt Tokens: 4\n",
-      "Completion Tokens: 38\n"
+      "Completion Tokens: 38\n",
+      "Successful Requests: 1\n"
      ]
     }
    ],
@@ -54,7 +55,8 @@
     "    result = llm(\"Tell me a joke\")\n",
     "    print(f\"Total Tokens: {cb.total_tokens}\")\n",
     "    print(f\"Prompt Tokens: {cb.prompt_tokens}\")\n",
-    "    print(f\"Completion Tokens: {cb.completion_tokens}\")"
+    "    print(f\"Completion Tokens: {cb.completion_tokens}\")\n",
+    "    print(f\"Successful Requests: {cb.successful_requests}\")"
    ]
   },
   {