Add Notion.Metrics for prometheus and telemetry metrics support

[coryodaniel]

Add Notion.Metrics for aggregating metrics from :telemetry.

Support:

• :prometheus type, defines prometheus, subscribes prometheus to telemetry, handle map to list for metadata
• :telemetry defines metrics, sets up supervisor

defmodule Bar.Instrumenter do
  use Notion, name: :bar
end

defmodule Bar.Metrics do
  use Notion.Metrics, type: :prometheus, instrumentor: Bar.Instrumenter
  
  defcounter [:http, :request], :duration, metadata: [:foo, :bar]
  deflast_value ...
  defsummary ...
  defhistogram [:http, :request], :duration, buckets: [100,500] ...
  defgauge # alias of deflast_value
end

s/tags/labels/metadata 🤷‍♂