Fix bug where tags list increases in size

[steven-liu]

Summary

When previously-defined metric-level tags (non-literal list) are passed into the gauge() function,
the metric-level tag list will be modified if constant_tags is not None. This is due
to the += operator modifying the metric-level list by appending
the list constant_tags.

If you're using dogstatsd to send your metrics, this will inevitably lead to a error: [Errno 90] Message too long since the UDP packet size has an upper limit.

For an example of this, see the new test, test_gauge_constant_tags_with_metric_level_tags_twice.

Fix

Use the following instead of +=:

# WILL modify metric-level tags passed into the function
tags += self.constant_tags

# WON'T modify metric-level tags passed into the function
tags = tags + self.constant_tags

[KyleJamesWalker]

+1

[yannmh]

Thanks for the fix @steven-liu !