Request to expose a configuration option to set headers for requests to APM server.

**Is your feature request related to a problem? Please describe.**
- I am using Node.js APM agent. And there is an AWS Api-gateway with API key placed between lambda APM agent and APM server. But there is no way to place the Api-gateway api key along with agent requests.

**Describe the solution you'd like**
- I need a way to tell the APM extension to add x-api-key: $configuredValue to all requests to APM server. I would like to request that the Node.js APM agent can expose a configuration option to set headers for requests to APM server.

**Describe alternatives you've considered**
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**Additional context**
- Elastic discuss: https://discuss.elastic.co/t/how-to-add-api-gateway-api-key-to-lambda-apm/317854