feat: add ELASTIC_APM_PAYLOAD_LOG debug option

Author: watson

README.md

@@ -190,6 +190,15 @@ Destroy the `client`. After this call, the client has ended and
 subsequent calls to `sendSpan()`, `sendTransaction()`, `sendError()`,
 `flush()`, or `end()` will result in an error.
 
+## Debugging
+
+To see what data is being sent to the APM Server, use the environment
+variable `ELASTIC_APM_PAYLOAD_LOG` to speicfy a log file, e.g:
+
+```
+ELASTIC_APM_PAYLOAD_LOG=/tmp/payload.ndjson
+```
+
 ## License
 
 MIT

index.js

@@ -77,6 +77,11 @@ function Client (opts) {
 
   pump(this._stream, this._chopper)
 
+  if (process.env.ELASTIC_APM_PAYLOAD_LOG) {
+    const logFile = require('fs').createWriteStream(process.env.ELASTIC_APM_PAYLOAD_LOG, {flags: 'a'})
+    pump(this._stream, logFile)
+  }
+
   this._index = clients.length
   clients.push(this)
 }