fix(span): Do not pass stack frames into promises

Passing error stack frames into a promise context makes it
uncollectable by the garbage collector.

lib/instrumentation/index.js

@@ -32,11 +32,9 @@ Instrumentation.prototype.start = function () {
     maxQueueSize: this._agent._conf.maxQueueSize
   }
   this._queue = new Queue(qopts, function onFlush (transactions, done) {
-    Promise.all(transactions).then(function (transactions) {
-      if (self._agent._conf.active && transactions.length > 0) {
-        request.transactions(self._agent, transactions, done)
-      }
-    }, done)
+    if (self._agent._conf.active && transactions.length > 0) {
+      request.transactions(self._agent, transactions, done)
+    }
   })
 
   if (this._agent._conf.asyncHooks && semver.gte(process.version, '8.2.0')) {
@@ -72,16 +70,13 @@ Instrumentation.prototype.addEndedTransaction = function (transaction) {
     debug('adding transaction to queue %o', {id: transaction.id})
     // encode the transaction as early as possible in an attempt to free up
     // objects for garbage collection
-    queue.add(new Promise(function (resolve, reject) {
-      transaction._encode(function (err, payload) {
-        // As of writing this comment, _encode swallows all errors. So the
-        // following error handling logic shouldn't actually be needed
-        if (err) reject(err)
-        else resolve(payload)
-      })
-    }).catch(function (err) {
-      logger.error('error encoding transaction %s: %s', transaction.id, err.message)
-    }))
+    transaction._encode(function (err, payload) {
+      if (err) {
+        logger.error('error encoding transaction %s: %s', transaction.id, err.message)
+        return
+      }
+      queue.add(payload)
+    })
   } else {
     debug('ignoring transaction %o', {id: transaction.id})
   }