Introduce copy-on-write for metrics in AST.

This depends on changes in:

https://github.com/prometheus/client_golang/tree/cow-metrics.

Change-Id: I80b94833a60ddf954c7cd92fd2cfbebd8dd46142
Add regression tests for metrics mutations in AST.

It turned out in the end, that only drop_common_metrics() produced any
erroneous output in the old system. The second expression in the test
("sum(testmetric) keeping_extra") already worked in the old code, but
why not keep it in...

The way to test ranged evaluations is a bit clumsy so far, so I want to
build a nicer test framework in the end, where all the test cases can be
specified as text files which specify desired inputs, outputs, query
step widths, etc.

Change-Id: I821859789e69b8232bededf670a1b76e9e8c8ca4
Merge pull request #420 from prometheus/metric-cow

Introduce copy-on-write for metrics in AST.