Refactor sinks to be stateful; Add a logger sink

[drinks]

Description

Currently this gem is only capable of writing to stdout via puts. I'd like to use it to emit from an EC2 instance into a log file, meaning I need to pass it an instance of a Logger to buffer output.

Changes

• Refactor sinks to be stateful
• Rename Lambda sink to Stdout for mental-model alignment with the new one (all sinks are now named after their output
  mechanism)
• Add Logger sink to accept any Logger instance and a level kwarg
• Beefed up test coverage around sinks

[metaskills]

This method is two lines long, did it need a paragraph break?

[drinks]

I was holding most of my lint tweaks back for the next PR but this one got lumped into the min stageable chunk. It's ruby style to follow any guard clause with an empty line: https://github.com/rubocop-hq/rubocop/blob/master/lib/rubocop/cop/layout/empty_line_after_guard_clause.rb

[metaskills]

That's cool. The node one is Console and Python is Lambda. So this means we are more like the node lib.

[metaskills]

So the pattern around the Node lib for CloudWatch on EC2 (https://github.com/awslabs/aws-embedded-metrics-python#configuration) is c.log_group_name and c.log_stream_name. Given that and knowing this is different, what do you think of this interface?

Aws::Embedded::Metrics.configure do |c|
  c.logger = Rails.logger
  c.logger_level = :info # This could be the default too.
  c.sink = :logger
end

I noticed this is very much how DHH likes to see things. Ex, when the attribute API was be written. Using symbols and objects vs having users construct object literals which might be private or refactored later. That way the sink getter or via a lookup_sink private helper looks for a symbol first. I'm fine either way.

[drinks]

🤔 would we anticipate that other sinks might have different initialization options? I'm open to this but it means either the footprint of attr_accessors for the config class grows with the product of each supported init signature or we end up with some method_missing / hash type thing. No dog in this fight for me, lemme know what you prefer.

[metaskills]

I'd say just merge it.