AddTags only respected when using metric sink routing

[akutta]

Context:

• https://github.com/stripe/veneur/pull/999/files
• add_tags replaced tags which changed the concept of adding tags to all metrics pre-sink, to allowing tags to be defined per sink

add_tags is only added if using metric sink routing is configured. This is a regression compared to the previous tags configuration that allowed all metrics to be tagged with additional tags whether the routing was enabled or not.