Phoenix Plugin: Correctly support multiple endpoints

[joshk]

Change description

The current logic for the keep_endpoint_metrics function doesn't work for cases where multiple endpoints are configured using the endpoints: key.

This change normalizes the endpoints config to work with module in phoenix_endpoints

What problem does this solve?

The events defined in the endpoint_info function weren't working as expected, as the keep_function_filter returned false. This started happening after I tested out the config for multiple endpoints.

Checklist

• My changes have passed unit tests and have been tested E2E in an example project.