feat(agent): return uuid in captureError callback (#636)

Fixes #573

Author: Qard

docs/agent-api.asciidoc

@@ -847,6 +847,8 @@ The optional `options` object can be used to log additional metadata with the er
 For details see the <<metadata,metadata section>>.
 
 The optional `callback` will be called after the error has been sent to the APM Server.
+It will receive an `Error` instance if the agent failed to send the error,
+and the id of the captured error.
 
 [[message-strings]]
 ===== Message strings

lib/agent.js

@@ -286,7 +286,9 @@ Agent.prototype.captureError = function (err, opts, cb) {
 
   function send (error) {
     agent.logger.info('logging error %s with Elastic APM', id)
-    request.errors(agent, [error], cb)
+    request.errors(agent, [error], (err) => {
+      if (cb) cb(err, error.id)
+    })
   }
 }
 

test/agent.js

@@ -260,10 +260,12 @@ test('#flush()', function (t) {
 
 test('#captureError()', function (t) {
   t.test('with callback', function (t) {
-    t.plan(5)
+    t.plan(7)
     APMServer()
       .on('listening', function () {
-        this.agent.captureError(new Error('with callback'), function () {
+        this.agent.captureError(new Error('with callback'), function (err, id) {
+          t.error(err)
+          t.ok(/^[a-z0-9-]*$/i.test(id), 'has valid error.id')
           t.ok(true, 'called callback')
           t.end()
         })