Add origin detection fields

[StephenWakely]

This enables origin detection.

• The value of the DD_EXTERNAL_ENV var is passed through to the e: field.
• Attempts to identify the container id are made by parsing various procfs files.
• Tag cardinality can be set via a global option, are passed to each individual metric. Cardinality is used by the agent to determine which tags to label the metric as per here.