[SLS-318] Add Ruby layer enhanced metrics

[agocs]

What does this PR do?

This adds the Invocations and Errors enhanced metrics to the Ruby lambda layer.

To be done:

• [x] naively include invocations and errors
• [x] Record all required tags per the architecture document
• [x] includes a check for DD_ENHANCED_METRICS
• [ ] ~respect DD_FLUSH_TO_LOG (needed?)~
• [ ] ~respect DD_SITE (needed?)~

Testing Guidelines

• [x] Added rspec tests to test behavior
• [x] More rspec for testing log output
• [x] Test in the Demo environment

Log output in the demo environment:

{"e":1580150180,"m":"aws.lambda.enhanced.invocations","t":["dd_lambda_layer:datadog-ruby25","functionname:hello-dog-ruby-dev-helloRuby25","region:us-east-1","account_id:172597598159","memorysize:128","cold_start:false","runtime:Ruby 2.5.7"],"v":1}
{"e":1580150180,"m":"aws.lambda.enhanced.errors","t":["dd_lambda_layer:datadog-ruby25","functionname:hello-dog-ruby-dev-helloRuby25","region:us-east-1","account_id:172597598159","memorysize:128","cold_start:false","runtime:Ruby 2.5.7"],"v":1}