fix: avoid copying label values from tsdb unless required

[sandeepsukhani]

What this PR does / why we need it:
We are seeing OOMKills on index gateways due to a huge amount of allocations from LabelValues method of tsdb index reader. It happens due to copying all the label values read from the tsdb. This problem arises when the label matchers have any negative matchers.

I have changed the code to avoid making a copy of the label values unless we want to send it back as a response. I have checked all the usages of that method, and the only place we need to copy the values is when the LabelValues method is called explicitly to get all the values.

Here is the benchmark showing the difference in performance:

benchmark                              old ns/op      new ns/op     delta
BenchmarkTSDBIndex_GetChunkRefs-10     875881104      927752604     +5.92%
BenchmarkTSDBIndex_GetChunkRefs-10     879311146      862689354     -1.89%
BenchmarkTSDBIndex_GetChunkRefs-10     881187479      926149750     +5.10%
BenchmarkTSDBIndex_GetChunkRefs-10     910079084      899049584     -1.21%
BenchmarkTSDBIndex_GetChunkRefs-10     1014711583     911985770     -10.12%

benchmark                              old allocs     new allocs     delta
BenchmarkTSDBIndex_GetChunkRefs-10     2000140        1000154        -50.00%
BenchmarkTSDBIndex_GetChunkRefs-10     2000139        1000151        -50.00%
BenchmarkTSDBIndex_GetChunkRefs-10     2000141        1000150        -50.00%
BenchmarkTSDBIndex_GetChunkRefs-10     2000141        1000149        -50.00%
BenchmarkTSDBIndex_GetChunkRefs-10     2000146        1000153        -50.00%

benchmark                              old bytes     new bytes     delta
BenchmarkTSDBIndex_GetChunkRefs-10     176039840     168119396     -4.50%
BenchmarkTSDBIndex_GetChunkRefs-10     176039752     168118520     -4.50%
BenchmarkTSDBIndex_GetChunkRefs-10     176039944     168118504     -4.50%
BenchmarkTSDBIndex_GetChunkRefs-10     176039952     168118408     -4.50%
BenchmarkTSDBIndex_GetChunkRefs-10     176057848     168119296     -4.51%

Special notes for your reviewer:
I introduced the copying of label values in PR #7502 to avoid corruption of values when the underlying tsdb is closed.

[cyriltovena]

LGTM