lib: add nsolidTracer EventEmitter

So we can subscribe to the `flagsUpdated` event to receive notifications
when the tracing flags have changed.
Use it in the otel/core.js:register() method.

PR-URL: #216
Reviewed-By: Rafael Gonzaga <[email protected]>
PR-URL: #245

Author: santigimeno

lib/internal/nsolid_trace.js

@@ -6,6 +6,8 @@ const {
   RegExp,
 } = primordials;
 
+const EventEmitter = require('events');
+const binding = internalBinding('nsolid_api');
 const { nsolidApi } = require('internal/async_hooks');
 
 const {
@@ -66,7 +68,14 @@ function extractSpanContextFromHttpHeaders(context, headers) {
   return getApi().trace.setSpanContext(context, spanContext);
 }
 
+const nsolidTracer = new EventEmitter();
+binding.setToggleTracingFn(() => {
+  nsolidTracer.emit('flagsUpdated');
+});
+
+
 module.exports = {
   extractSpanContextFromHttpHeaders,
   generateSpan,
+  nsolidTracer,
 };

lib/internal/otel/core.js

@@ -26,8 +26,9 @@ function register(api) {
   // TODO(santigimeno): perform some kind of validation that the api is actually
   // the OTEL api.
   api_ = api;
-  binding.setToggleTracingFn((enable) => {
-    if (enable) {
+  const { nsolidTracer } = require('internal/nsolid_trace');
+  nsolidTracer.on('flagsUpdated', () => {
+    if (binding.trace_flags[0]) {
       enableApi();
     } else {
       disableApi();