Fix fetched chunk from store size in metric (#8971)

**What this PR does / why we need it**:
The `Size()` function on `chunk.Chunk` calls out to `func (ChunkRef)
Size() int` in the proto, which doesn't actually include the size of the
chunk data! We need to call `chunk.Data.Size()` for that.

CHANGELOG.md

@@ -7,6 +7,10 @@
 * [8953](https://github.com/grafana/loki/pull/8953) **dannykopping**: Querier: block queries by hash.
 * [8851](https://github.com/grafana/loki/pull/8851) **jeschkies**: Introduce limit to require a set of labels for selecting streams.
 
+##### Fixes
+
+* [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.
+
 ### All Changes
 
 #### Promtail

pkg/storage/chunk/fetcher/fetcher.go

@@ -201,9 +201,10 @@ func (c *Fetcher) FetchChunks(ctx context.Context, chunks []chunk.Chunk, keys []
 	// to the cache asynchronously in the background and we lose the context
 	var bytes int
 	for _, c := range fromStorage {
-		bytes += c.Size()
+		size := c.Data.Size()
+		bytes += size
 
-		chunkFetchedSize.WithLabelValues("store").Observe(float64(c.Size()))
+		chunkFetchedSize.WithLabelValues("store").Observe(float64(size))
 	}
 
 	st := stats.FromContext(ctx)