Statsd custom interpolation

There was a collision between heira and logship's statsd shipper doing
variable interpolation. To allow a change of pattern I have added a
pattern argument which can be passed in the statsd shipper description,
as follows:

statsd_counter,pattern=<some_pattern>,metric=blah.<use_of_pattern>.blah

If this parameter is not defined it will default to the previously used
pattern.