chore: add payloadLogFile debug config option (#500)

Author: watson

README.md

@@ -47,8 +47,13 @@ npm install elastic-apm-node --save
 Contributions are welcome,
 but we recommend that you take a moment and read our [contribution guide](CONTRIBUTING.md) first.
 
-To ease development,
-set the environment variable `DEBUG_PAYLOAD=1` to have the agent dump the JSON payload sent to the APM Server to a temporary file on your local harddrive.
+To see what data is being sent to the APM Server,
+use the environment variable `ELASTIC_APM_PAYLOAD_LOG_FILE` (or the config option `payloadLogFile`) to speicfy a log file,
+e.g:
+
+```
+ELASTIC_APM_PAYLOAD_LOG_FILE=/tmp/payload.ndjson
+```
 
 Please see [TESTING.md](TESTING.md) for instructions on how to run the test suite.
 

lib/agent.js

@@ -156,7 +156,10 @@ Agent.prototype.start = function (opts) {
 
     // Streaming conf
     size: this._conf.apiRequestSize,
-    time: this._conf.apiRequestTime * 1000
+    time: this._conf.apiRequestTime * 1000,
+
+    // Debugging
+    payloadLogFile: this._conf.payloadLogFile
   })
   this._apmServer.on('error', err => {
     this.logger.error('An error occrued while communicating with the APM Server:', err.message)

lib/config.js

@@ -88,7 +88,8 @@ var ENV_TABLE = {
   transactionMaxSpans: 'ELASTIC_APM_TRANSACTION_MAX_SPANS',
   transactionSampleRate: 'ELASTIC_APM_TRANSACTION_SAMPLE_RATE',
   serverTimeout: 'ELASTIC_APM_SERVER_TIMEOUT',
-  disableInstrumentations: 'ELASTIC_APM_DISABLE_INSTRUMENTATIONS'
+  disableInstrumentations: 'ELASTIC_APM_DISABLE_INSTRUMENTATIONS',
+  payloadLogFile: 'ELASTIC_APM_PAYLOAD_LOG_FILE'
 }
 
 var BOOL_OPTS = [