Copy label values to avoid unsafe memory use

[bboreham]

This is an alternative solution to #2000 (and #2004) where we do more copying on the error path but need fewer comments to avoid mistakes.

Benchmarks show this alternative is significantly more expensive on the error path:
Before #2000

BenchmarkIngesterPush/encoding=DoubleDelta	   20165041 ns/op  2590736 B/op	   12441 allocs/op
BenchmarkIngesterPushErrors/encoding=DoubleDelta   26534724 ns/op  5270153 B/op	   85194 allocs/op

After #2000

BenchmarkIngesterPush/encoding=DoubleDelta	   20305149 ns/op  2591191 B/op	   12444 allocs/op
BenchmarkIngesterPushErrors/encoding=DoubleDelta   27959445 ns/op  5276005 B/op	   85189 allocs/op

This branch:

BenchmarkIngesterPush/encoding=DoubleDelta	   20074468 ns/op  2591331 B/op	   12598 allocs/op
BenchmarkIngesterPushErrors/encoding=DoubleDelta   35415601 ns/op 11072246 B/op	   95055 allocs/op

[pracucci]

@bboreham Do you mind rebasing? This stuff has changed in the WAL PR #1103 which has just been merged 😞

[bboreham]

I think the benchmarks show that we don't want to do this so have updated #2004.