Add instructions for manual tracer initialization (#122)

* Add instructions for manual tracer initialization

* Update README.md

Co-authored-by: Stephen Firrincieli <[email protected]>

Co-authored-by: Stephen Firrincieli <[email protected]>

Author: tianchu

README.md

@@ -25,8 +25,26 @@ For additional details on trace collection, take a look at [collecting traces fr
 
 For additional details on trace and log connection, check out the [official documentation for Datadog trace client](https://datadoghq.dev/dd-trace-js/).
 
+### Plugins
+
 The `fs` module is disabled by default. If you want to enable it you have to set the environment variable `DD_TRACE_DISABLED_PLUGINS` to `''` or to a comma-separated list of the plugins you want to disable. See the full list of supported plugins [here](https://docs.datadoghq.com/tracing/compatibility_requirements/nodejs/).
 
+### Tracer Initialization
+
+By default, the Datadog Lambda library automatically initializes the tracer. However, you can follow the steps below to initialize the tracer with [custom settings](https://datadoghq.dev/dd-trace-js/#tracer-settings) in your own function code.
+
+1. Set enviornment variable `DD_TRACE_ENABLED` to `false`, so the Datadog Lambda library does not initialize the tracer.
+1. Add the following snippet to the function code to manually initialize the tracer with your desired settings.
+    ```js
+    const tracer = require('dd-trace').init({
+      enabled: true,
+      tags: {
+        "_dd.origin": "lambda",
+      },
+      sampleRate: 0.1 // e.g., keep 10% of traces
+    });
+    ```
+
 ### Trace & Log Correlation
 
 By default, the Datadog trace id gets automatically injected into the logs for correlation, if using `console` or a logging library supported for automatic trace id injection. You have to manually inject the trace id, if using other logging libraries. See additional details on [connecting logs and traces](https://docs.datadoghq.com/tracing/connect_logs_and_traces/nodejs/).