Current restrictions for tags on transactions:

• Tag keys have a maximum length of 32 characters and can contain only letters (a-zA-Z), numbers (0-9), underscores (_), periods (.), colons (:), and dashes (-).
• Tag values have a maximum length of 200 characters and they cannot contain the newline (\n) character.

Current restrictions in metrics are different (spec)

To do:
a) extend statsd protocol to support more characters
b) update sdks
c) update spec
d) enforce matching max limits on tag keys and values if necessary for correlation