chore: Update end to end tracing documentation

OBSERVABILITY.md

@@ -88,8 +88,9 @@ const spanner = new Spanner({
 
 In addition to client-side tracing, you can opt in for end-to-end tracing. End-to-end tracing helps you understand and debug latency issues that are specific to Spanner. Refer [here](https://cloud.google.com/spanner/docs/tracing-overview) for more information.
 
-You can opt-in by either:
+To configure end-to-end tracing.
 
+1. Opt in for end-to-end tracing. You can opt-in by either:
 * Setting the environment variable `SPANNER_ENABLE_END_TO_END_TRACING=true` before your application is started
 * In code, setting `enableEndToEndTracing: true` in your SpannerOptions before creating the Cloud Spanner client
 
@@ -103,6 +104,13 @@ const spanner = new Spanner({
 }),
 ```
 
+2. Set the trace context propagation in OpenTelemetry.
+```javascript
+const {propagation} = require('@opentelemetry/api');
+const {W3CTraceContextPropagator} = require('@opentelemetry/core');
+propagation.setGlobalPropagator(new W3CTraceContextPropagator());
+```
+
 #### OpenTelemetry gRPC instrumentation
 
 Optionally, you can enable OpenTelemetry gRPC instrumentation which produces traces of executed remote procedure calls (RPCs)

samples/observability-traces.js

@@ -41,6 +41,11 @@
 });
 provider.addSpanProcessor(new BatchSpanProcessor(exporter));
 
+// Set global propagator to propogate the trace context for end to end tracing.
+const {propagation} = require('@opentelemetry/api');
+const {W3CTraceContextPropagator} = require('@opentelemetry/core');
+propagation.setGlobalPropagator(new W3CTraceContextPropagator());
+
 // Uncomment following line to register global tracerProvider instead
 // of passing it into SpannerOptions.observabilityOptions.
 // provider.register();