Allow ignoring the requests Context header (#295)

* Allow ignoring the requests Context header

* added simple test for the header disabling config flag

* fixed test

config.js

@@ -64,7 +64,7 @@ module.exports = {
 
     // Specifies the behavior of the trace agent in the case of an uncaught exception.
     // Possible values are:
-    //   `ignore`: Take no action. Note that the process may termiante before all the 
+    //   `ignore`: Take no action. Note that the process may termiante before all the
     //            traces currently buffered have been flushed to the network.
     //   `flush`: Handle the uncaught exception and attempt to publish the traces to
     //            the API. Note that if you have other uncaught exception handlers in your
@@ -79,6 +79,9 @@ module.exports = {
     //            a delay. Note that presence of other uncaught exception handlers may
     //            chose to terminate the application before the buffer has been flushed to
     //            the network.
-    onUncaughtException: 'ignore'
+    onUncaughtException: 'ignore',
+
+    // Allows to ignore the requests X-Cloud-Trace-Context -header if set
+    ignoreContextHeader: false
   }
 };

lib/trace-agent.js

@@ -231,6 +231,9 @@ TraceAgent.prototype.isTraceAgentRequest = function(options) {
  *         object with keys. null if there is a problem.
  */
 TraceAgent.prototype.parseContextFromHeader = function(str) {
+  if (this.config_.ignoreContextHeader) {
+    return null;
+  }
   if (!str) {
     return null;
   }

test/test-trace-agent.js

@@ -160,6 +160,16 @@ describe('Trace Agent', function() {
         });
       });
     });
+
+    describe('when configured to ignore header', function() {
+      it('should return expected value: null', function() {
+        agent.config_.ignoreContextHeader = true;
+        var result = agent.parseContextFromHeader(
+          '123456/667;o=1');
+        assert(!result);
+        agent.config_.ignoreContextHeader = false;
+      });
+    });
   });
 
 });