Add phoenix logger w/ telemetry events

[StephaneRob]

Add ability to use telemetry events to log phoenix requests. This logger has the following options:

• metadata_formatter default to LoggerJSON.Plug.MetadataFormatters.GoogleCloudLogger
• version_header default to x-api-version
• unit (only for datadog for now) to choose the unit of the duration.

[StephaneRob]

Hey @AndrewDryga, any thoughts on this? The idea is to provide a telemetry version of LoggerJSON.Plug

[AndrewDryga]

Closing this for now as the library was rewritten this week to be an Erlang :logger formatted.

In the upcoming version we don't provider plugs or telemetry attaches (because that should be handled by metrics collection anyway), but anyone can attach request metadata by just adding conn to the metadata: Logger.log(..., ..., conn: conn).