Add filtering support for metadata payload

In Kibana’s APM instrumentation, we are now capturing stats from developer’s local environments. One issue is that we cannot filter some PII such as the developer’s username from the path that is contained in the process.args array in the metadata that is sent along with APM traces:

Currently, we’re including a notice that some personal data may be captured with an option for disabling this. Ideally, we’d be able filter this out similar to how we can filter out transaction data.

[trentm]

@tylersmalley Thanks! You are right. process.title is the other one that can hold the full path. I’m sold on the use case then. I’ll get a PR moving for adding a metadata filter, then.

[trentm]

Some quick notes and a first hack:

1. patch to apm-nodejs-http-client.git

diff --git a/index.js b/index.js
index c27fb71..2bb3a0d 100644
--- a/index.js
+++ b/index.js
@@ -495,7 +495,9 @@ function onStream (client, onerror) {

     // All requests to the APM Server must start with a metadata object
     if (!client._encodedMetadata) {
-      client._encodedMetadata = client._encode({ metadata: client._conf.metadata }, Client.encoding.METADATA)
+      // XXX HACK: This should be done *once*, but as late as possible.
+      var filteredMetadata = client._conf.metadataFilters.process(client._conf.metadata)
+      client._encodedMetadata = client._encode({ metadata: filteredMetadata }, Client.encoding.METADATA)
     }
     stream.write(client._encodedMetadata)
   }

1. patch to apm-agent-nodejs.git

diff --git a/lib/agent.js b/lib/agent.js
index cd75056..56038c1 100644
--- a/lib/agent.js
+++ b/lib/agent.js
@@ -42,6 +42,7 @@ function Agent () {
   this._errorFilters = new Filters()
   this._transactionFilters = new Filters()
   this._spanFilters = new Filters()
+  this._metadataFilters = new Filters()
   this._transport = null

   this.lambda = lambda(this)
@@ -221,6 +224,12 @@ Agent.prototype.addFilter = function (fn) {
   this.addErrorFilter(fn)
   this.addTransactionFilter(fn)
   this.addSpanFilter(fn)
+  // XXX Decide if this would be a breaking change. For example the *default
+  // filter example in the docs* will break because it assumes
+  // `payload.context`. However that should *already* break for filtering
+  // error payloads.
+  // https://www.elastic.co/guide/en/apm/agent/nodejs/current/agent-api.html#apm-add-filter
+  this.addMetadataFilter(fn)
 }

 Agent.prototype.addErrorFilter = function (fn) {
@@ -250,6 +259,15 @@ Agent.prototype.addSpanFilter = function (fn) {
   this._spanFilters.push(fn)
 }

+Agent.prototype.addMetadataFilter = function (fn) {
+  if (typeof fn !== 'function') {
+    this.logger.error('Can\'t add filter of type %s', typeof fn)
+    return
+  }
+
+  this._metadataFilters.push(fn)
+}
+
 Agent.prototype.captureError = function (err, opts, cb) {
   if (typeof opts === 'function') return this.captureError(err, null, opts)

diff --git a/lib/config.js b/lib/config.js
index 2702181..e69e6f5 100644
--- a/lib/config.js
+++ b/lib/config.js
@@ -253,6 +253,7 @@ class Config {
           globalLabels: maybePairsToObject(conf.globalLabels),
           hostname: conf.hostname,
           environment: conf.environment,
+          metadataFilters: agent._metadataFilters,

           // Sanitize conf
           truncateKeywordsAt: config.INTAKE_STRING_MAX_SIZE,

1. which allows me to write a (poor) filter somewhat for your case like this:

apm.addMetadataFilter(function myFilt(payload) {
  if (payload.process && payload.process.argv) {
    const user = new RegExp(process.env.USER, 'g')
    payload.process.argv = payload.process.argv.map((arg) => {
      return arg.replace(user, '[REDACTED]')
    })
  }
  return payload
})

and then observe that filtering in traffic to apm-server:

[2020-12-15T22:33:03.579Z]  INFO: mockapmserver/26715 on pink.local: request (req.remoteAddress=::ffff:127.0.0.1, req.remotePort=60675)
    POST /intake/v2/events HTTP/1.1
    accept: application/json
    user-agent: elasticapm-node/3.9.0 elastic-apm-http-client/9.4.2 node/10.23.0
    content-type: application/x-ndjson
    content-encoding: gzip
    host: localhost:8200
    connection: keep-alive
    transfer-encoding: chunked

    {"metadata":{"service":{"name":"esapp","environment":"development","runtime":{"name":"node","version":"10.23.0"},"language":{"name":"javascript"},"agent":{"name":"nodejs","version":"3.9.0"},"framework":{"name":"express","version":"4.17.1"},"version":"1.0.1"},"process":{"pid":28461,"ppid":27276,"title":"node","argv":["/Users/[REDACTED]/.nvm/versions/node/v10.23.0/bin/node","/Users/[REDACTED]/tm/play/esapp.js"]},"system":{"hostname":"pink.local","architecture":"x64","platform":"darwin"}}}
    {"span":{"name":...