Add docs for tracing environment variable (#4477)

langchain/experimental/client/tracing_datasets.ipynb

@@ -77,7 +77,9 @@
     "\n",
     "Datasets can be created in a number of ways, most often by collecting `Run`'s captured through the LangChain tracing API and converting a set of runs to a dataset.\n",
     "\n",
-    "The V2 tracing API is currently accessible using the `tracing_v2_enabled` context manager. Assuming the server was succesfully started above, running LangChain Agents, Chains, LLMs, and other primitives will then automatically capture traces. We'll start with a simple math example."
+    "The V2 tracing API is currently accessible using the `tracing_v2_enabled` context manager. Assuming the server was succesfully started above, running LangChain Agents, Chains, LLMs, and other primitives will then automatically capture traces. We'll start with a simple math example.\n",
+    "\n",
+    "**Note** You can also use the `LANGCHAIN_TRACING_V2` environment variable to enable tracing for all runs by default, regardless of whether or not those runs happen within the `tracing_v2_enabled` context manager (i.e. `os.environ[\"LANGCHAIN_TRACING_V2\"] = \"true\"`)"
    ]
   },
   {
@@ -975,7 +977,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.11.2"
+   "version": "3.10.9"
   }
  },
  "nbformat": 4,