Proxy configuration from Kibana to agents

[jalvz]

Initial requirements

• Fetch configuration changes from a Kibana REST API
• Serve configuration to agents with an Etag header
• Support granularity per service name and environment
• Minimize within reason the number of requests to Kibana and the response time for agents

Background

• It has been considered to fetch configuration directly form Elasticsearch
• It has been considered to poll Kibana for all configuration at regular intervals

Subtasks

• Main functionality Adds initial support for agent remote configuration #2289
• Beats support Adapt Kibana interface for external usage beats#12266
• Performance [ACM] Optimization / caching #2220
• Integration tests [ACM] Add Integration tests #2219
• Server Docs [docs] APM Agent configuration documentation apm#86

Parent meta-issue

• Design: APM Server should be able to proxy Central Configuration to Agents apm#4

[jalvz]

POC: #2095