Refactor Prebid-Server to be a more flexible platform

[scr-oath]

Context

Companies who host their own prebid server may need to configure, customize, and extend it for their own needs.

At the moment, the main method and its helpers are all private, and every adapter, module, analytics is decided by PBS.

This proposal (a placeholder requested during the Server PMC) would be to refactor the code to be used a as a library. Some reference libraries/examples are http.Server, or for the powerful function options pattern, the open-telemetry library (TracerProvider, e.g.)

The end result and goals are that hosting providers be able to:

1. Configure the server
2. Launch the server
3. Mock the launch for testing
4. Add modules
5. Add adapters
6. Add analytics
7. Add MetricsEngines and choose them
8. Add more endpoints - or just get the configured httprouter and use that.

[bretg]

Discussed in committee

• The ask is to allow PBS-core platform to be way more configurable. Use cases:
  • allowing the addition of bid adapters and analytics adapters
  • adding metrics engines
  • add modules
  • add telemetry engine
  • add storage backends
  • easier to add custom endpoints
• e.g. for Go, export server start routine and make it overridable
• One idea is that host companies could re-write main and pull in their own components.

Next steps:

• @scr-oath is willing to share a proof-of-concept in a PR for the PBS-Go team to review
• Continue this conversation on #prebid-server-dev Slack channel