Estimate chunk size on compaction looking at data uncompressed size (#9099)

**What this PR does / why we need it**:
At `compactedIndex.ToIndexFile` we were not computing the size of the
chunk correctly. We were looking at the size of the chunk ref. Since the
size of the chunk ref is always a bunk of bytes, the chunk meta KB field
would always be 0.

4e893a0a88/pkg/storage/stores/tsdb/compactor.go (L367)

Instead, we should get the approx KBs by looking at the chunks data
uncompressed size. As we do at `store.IndexChunk`:

d405162516/pkg/storage/stores/tsdb/store.go (L189)

CHANGELOG.md

@@ -12,6 +12,7 @@
 
 * [8971](https://github.com/grafana/loki/pull/8971) **dannykopping**: Stats: fix `Cache.Chunk.BytesSent` statistic and loki_chunk_fetcher_fetched_size_bytes metric with correct chunk size.
 * [8979](https://github.com/grafana/loki/pull/8979) **slim-bean**: Fix the case where a logs query with start time == end time was returning logs when none should be returned.
+* [9099](https://github.com/grafana/loki/pull/9099) **salvacorts**: Fix the estimated size of chunks when writing a new TSDB file during compaction.
 
 ### All Changes
 

pkg/storage/stores/tsdb/compactor.go

@@ -360,11 +360,12 @@ func (c *compactedIndex) ToIndexFile() (index_shipper.Index, error) {
 		b.Del(labels.MetricName)
 		ls := b.Labels(nil)
 
+		approxKB := math.Round(float64(chk.Data.UncompressedSize()) / float64(1<<10))
 		err := c.builder.InsertChunk(ls.String(), index.ChunkMeta{
 			Checksum: chk.Checksum,
 			MinTime:  int64(chk.From),
 			MaxTime:  int64(chk.Through),
-			KB:       uint32(chk.Size()) / (1 << 10),
+			KB:       uint32(approxKB),
 			Entries:  uint32(chk.Data.Entries()),
 		})
 		if err != nil {

pkg/storage/stores/tsdb/compactor_test.go

@@ -202,6 +202,7 @@ func buildChunkMetas(from, to int64) index.ChunkMetas {
 			MaxTime:  i + 1,
 			Checksum: uint32(i),
 			Entries:  1,
+			KB:       1,
 		})
 	}
 
@@ -897,6 +898,10 @@ type dummyChunkData struct {
 	chunk.Data
 }
 
+func (d dummyChunkData) UncompressedSize() int {
+	return 1 << 10 // 1KB
+}
+
 func (d dummyChunkData) Entries() int {
 	return 1
 }