Improve metric queries by computing samples at the edges.

[cyriltovena]

This PR pushes the metric extraction/transformation to the edges (ingester/storage) instead of doing it in the engine. This allow to create metric without making a string allocation from the line buffer while decompressing which reduce drastically memory allocations, and speed up those metric queries.

I have observed 2x improvement for all metric queries. Deduping of log line is done by using a hash of the log line and not the content anymore, I'm using xxhash which has shown very good performance and few chances of collisions see https://github.com/Cyan4973/xxHash.

Another interesting changes, the store now implement chunk.Store and logql.Querier this makes it easier to use it with the LogQL engine.

The PR is big as it splits the whole code base in 2.

Next possible steps:

• Introduce a Seek function in the SampleIterator allowing range vector iterator to skip through lazy chunk and blocks. Specially when doing query where the range is smaller than the step e.g rate({app="foo"}[1m]) with a step of 5m
• Try to merge those duplicated logic, I did intentionally not pushed this too much to avoid refactoring of what was already existing. But some complex logic has been reused such as the batch iterator.
• There's some places where we could reuse slice, specially caches, but this is big enough.

When deploying this change, ingester should be fully roll out first, as it introduces a new GRPC service for requesting sample to ingester.

I really wanted to get this change in before we introduce LogQL v2, I believe now is easier. Again I'm sorry for the big PR.

[codecov-commenter]

Codecov Report

Merging #2293 into master will increase coverage by 0.26%.
The diff coverage is 70.58%.

@@            Coverage Diff             @@
##           master    #2293      +/-   ##
==========================================
+ Coverage   62.29%   62.55%   +0.26%     
==========================================
  Files         158      159       +1     
  Lines       12766    13361     +595     
==========================================
+ Hits         7952     8358     +406     
- Misses       4201     4363     +162     
- Partials      613      640      +27     

Impacted Files	Coverage Δ
pkg/chunkenc/dumb_chunk.go	0.00% <0.00%> (ø)
pkg/chunkenc/interface.go	87.50% <ø> (ø)
pkg/ingester/stream.go	73.80% <0.00%> (-3.70%)	⬇️
pkg/iter/entry_iterator.go	67.97% <0.00%> (ø)
pkg/logcli/query/query.go	0.00% <0.00%> (ø)
pkg/logproto/types.go	46.89% <ø> (ø)
pkg/ingester/instance.go	53.95% <5.12%> (-8.29%)	⬇️
pkg/ingester/ingester.go	51.20% <20.45%> (-8.45%)	⬇️
pkg/querier/querier.go	63.26% <28.57%> (-8.07%)	⬇️
pkg/logql/sharding.go	59.48% <33.33%> (-0.31%)	⬇️
... and 15 more

[slim-bean]

More great work @cyriltovena !! Great work with tests!