Wrote a small test program to test this based on the dev setup described in elastic/apm#76 (comment):

const Client = require('elastic-apm-http-client')

const client = new Client({
  serviceName: 'opbeans-fortran',
  agentName: 'my-nodejs-agent',
  agentVersion: '1.0.0',
  userAgent: 'My Custom Elastic APM Agent',
  remoteConfig: true
})

client.on('config', function (conf) {
  console.log('new config:', conf)
})

client.on('request-error', function (err) {
  console.log('request-error:', err)
})

setTimeout(function () {}, 100000) // don't exit the process

This was the console output:

-- polling
-- request: {
  agent: undefined,
  rejectUnauthorized: true,
  hostname: 'localhost',
  port: '8200',
  method: 'GET',
  path: '/config/v1/agents?service.name=opbeans-fortran&environment=development',
  headers: {
    Accept: 'application/json',
    'User-Agent': 'My Custom Elastic APM Agent elastic-apm-http-client/8.0.0'
  }
}
-- got poll response (status: 200): {
  'cache-control': 'max-age=30, must-revalidate',
  'content-type': 'application/json',
  etag: '5',
  date: 'Fri, 05 Jul 2019 10:20:36 GMT',
  'content-length': '30',
  connection: 'close'
}
-- sechedule poll in 30 seconds
new config: { sampling_rate: '0.12' }
-- polling
-- request: {
  agent: undefined,
  rejectUnauthorized: true,
  hostname: 'localhost',
  port: '8200',
  method: 'GET',
  path: '/config/v1/agents?service.name=opbeans-fortran&environment=development',
  headers: {
    Accept: 'application/json',
    'User-Agent': 'My Custom Elastic APM Agent elastic-apm-http-client/8.0.0',
    'If-None-Match': '5'
  }
}
-- got poll response (status: 304): {
  'cache-control': 'max-age=30, must-revalidate',
  etag: '5',
  date: 'Fri, 05 Jul 2019 10:21:06 GMT',
  connection: 'close'
}
-- sechedule poll in 30 seconds

@Qard just added two commits, the primary one to just add a notice about required APM Server version (the other was just code cleanup I snug in since I had to push the doc change)

Update: Had to revert that last commit, as default function args doesn't work exactly how falsy values does. In our case, there's a risk at the number of seconds given is NaN in case the max-age cannot be parsed. And we want to fall back to 300 seconds in that case.

... aaand another one. Just found a "typo" in the new docs that I wanted to fix.

I just updated the PR with the latest updates from elastic/apm#76:

• The APM Server no longer responds with 500 on the config endpoint. Instead, it responds with:
  • 403 if the APM Server isn't configured to expose /config/v1/agents
  • 503 if the APM Server can't reach Kibana
• It seems like we're settling on centralConfig instead of remoteConfig. I've updated the PR in anticipation of this (hopefully not prematurely)

It also seems like we're going to decide to have this feature enabled by default. I still think we should leave it off by default in the http client, no matter if we end up turning it on by default in the agent. This will allow us to release this PR to npm as a minor bump in good time before 7.3 without worrying about anything.