Allow user to pass a lambda function to dynamic set the tags

[rafaelzimmermann]

Adds the same functionality that allows dynamic metric names to tags. Now you can use lambda function that instead of a list of tags to set the metric tags. The lambda function must accept two arguments:

• the object the function is being called on
• the array of arguments passed

This is useful when defining metaprogramming methods.

Comparison:
UDP batched (branch: dynamic_tags, sha: d650250):   264181.0 i/s
UDP batched (branch: master, sha: e69ab35):   256938.6 i/s - same-ish: difference falls within error