Don't keep extra labels in aggregations by default.

MIN/MAX/SUM/AVG/COUNT aggregations will now by default drop all labels that are
not specifically part of a BY-clause, even if a label value is the same within
all timeseries of an aggregation group. The old behavior of keeping extra
labels may still be switched on by adding KEEPING_EXTRA to the end of an
aggregation statement:

  sum(http_requests) by (job, method) keeping_extra

I'm open to better syntax/naming suggestions.

Change-Id: I21d3fe7af9e98552ce3dffa3ce7c0a4ba4c0b4a4
Merge "Don't keep extra labels in aggregations by default."