[META 355] Create Spec and POC for APM agents integration with Prometheus

[AlexanderWert]

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This issue covers the creation of a POC for an integration of APM agents with the Prometheus client libraries.

Detailed description on the goal: https://github.com/elastic/apm/issues/355

This issue is considered as completed when the following is available:

• POC with one APM agent
• Spec describing a language- / agent-independent approach of integrating with Prometheus client libraries

[AlexanderWert]

related: https://github.com/elastic/apm-agent-python/issues/972