feat(js): Adds docs on tracing in serverless environments in JS (#492)

Author: jacoblee93

docs/observability/how_to_guides/index.md

@@ -34,6 +34,7 @@ Set up LangSmith tracing to get visibility into your production applications.
 - [Prevent logging of sensitive data in traces](./how_to_guides/tracing/mask_inputs_outputs)
 - [Trace generator functions](./how_to_guides/tracing/trace_generator_functions)
 - [Calculate token-based costs for traces](./how_to_guides/tracing/calculate_token_based_costs)
+- [Trace JS functions in serverless environments](./how_to_guides/tracing/serverless_environments)
 
 ## Tracing projects UI & API
 

docs/observability/how_to_guides/tracing/serverless_environments.mdx

@@ -0,0 +1,37 @@
+# Trace JS functions in serverless environments
+
+:::note
+This section is relevant for those using the LangSmith JS SDK version 0.2.0 and higher.
+If you are tracing using LangChain.js or LangGraph.js in serverless environments, see [this guide](https://js.langchain.com/docs/how_to/callbacks_serverless).
+:::
+
+When tracing JavaScript functions, LangSmith will trace runs in the background by default to avoid adding latency.
+In serverless environments where the execution context may be terminated abruptly, it's important to ensure that all tracing data is properly flushed before the function completes.
+
+To make sure this occurs, you can either:
+
+- Set an environment variable named `LANGSMITH_TRACING_BACKGROUND` to `"false"`. This will cause your traced functions to wait for tracing to complete before returning.
+  - Note that this is named differently from the [environment variable](https://js.langchain.com/docs/how_to/callbacks_serverless) in LangChain.js because LangSmith can be used without LangChain.
+- Pass a custom client into your traced runs and `await` the `client.awaitPendingTraceBatches();` method.
+
+Here's an example of using `awaitPendingTraceBatches` alongside the [`traceable`](/observability/how_to_guides/tracing/annotate_code) method:
+
+```ts
+import { Client } from "langsmith";
+import { traceable } from "langsmith/traceable";
+
+const langsmithClient = new Client({});
+
+const tracedFn = traceable(
+  async () => {
+    return "Some return value";
+  },
+  {
+    client: langsmithClient,
+  }
+);
+
+const res = await tracedFn();
+
+await langsmithClient.awaitPendingTraceBatches();
+```

docs/observability/how_to_guides/tracing/trace_with_langchain.mdx

@@ -281,7 +281,7 @@ console.log(runId);`),
 
 In LangChain Python, LangSmith's tracing is done in a background thread to avoid obstructing your production application. This means that your process may end before all traces are successfully posted to LangSmith. This is especially prevalent in a serverless environment, where your VM may be terminated immediately once your chain or agent completes.
 
-In LangChain JS/TS, the default is to block for a short period of time for the trace to finish due to the greater popularity of serverless environments. You can make callbacks asynchronous by setting the `LANGCHAIN_CALLBACKS_BACKGROUND` environment variable to `"true"`.
+You can make callbacks synchronous by setting the `LANGCHAIN_CALLBACKS_BACKGROUND` environment variable to `"false"`.
 
 For both languages, LangChain exposes methods to wait for traces to be submitted before exiting your application.
 Below is an example:

versioned_docs/version-old/tracing/faq/langchain_specific_guides.mdx

@@ -187,7 +187,7 @@ This tactic is also useful for when you have multiple chains running in a shared
 
 In LangChain Python, LangSmith's tracing is done in a background thread to avoid obstructing your production application. This means that your process may end before all traces are successfully posted to LangSmith. This is especially prevalent in a serverless environment, where your VM may be terminated immediately once your chain or agent completes.
 
-In LangChain JS, prior to `@langchain/core` version `0.3.0`, the default was to block for a short period of time for the trace to finish due to the greater popularity of serverless environments. Versions `>=0.3.0` will have the same default as Python.
+In LangChain JS, prior to `@langchain/core` version `0.3.0`, the default was to block for a short period of time for the trace to finish due to the greater popularity of serverless environments. Versions `>=0.3.0` have the same default as Python.
 You can explicitly make callbacks synchronous by setting the `LANGCHAIN_CALLBACKS_BACKGROUND` environment variable to `"false"` or asynchronous by setting it to `"true"`. You can also check out [this guide](https://js.langchain.com/docs/how_to/callbacks_serverless) for more options for awaiting backgrounded callbacks in serverless environments.
 
 For both languages, LangChain exposes methods to wait for traces to be submitted before exiting your application.