v3.1.0 memory leak when metricsInterval is 0

Describe the bug Memory leak detected when metricsInterval is set 0. You just start application and wait and then take a heap dump.

Repository: https://github.com/evgeni-k/apm-node-metrics-mem-leak

Config:

const APM = require(`elastic-apm-node`);
APM.start({
    metricsInterval: 0,
    metricsLimit: 100,
    serviceName: `mem-leak`,
    serverUrl: `http://localhost:8200`,
});

Production memory consumption graph:

Local heapdumps:

To Reproduce

1. start agent with metricsInterval 0.
2. wait
3. take a heap dump and analyze Metrics usage

Expected behavior

If set up to not send metrics then it will be great to not collect them at all.

Environment (please complete the following information)

• OS: Win10 x64
• Node.js 10.15.0
• APM Server: 7.4
• Agent vresion: 3.1.0

How are you starting the agent? (please tick one of the boxes)

• Calling agent.start() directly (e.g. require('elastic-apm-node').start(...))
• Requiring elastic-apm-node/start from within the source code
• Starting node with -r elastic-apm-node/start

[axw]

I’ve merged a fix - new release coming soon!

[axw]

The fix has been released in v3.2.0.