Increase the chunk_retain_period to 15 minutes

This increases the time a chunk has to be filled before being flushed.
With shorter times, it could happen that during times of low traffic
chunks will not be filled completely before being flushed. This would
lead to small chunk objects, which is inefficient.

Change-Id: I74b2af1a053c8d4298b9e9d7ffca04cb9d8926bd
diff --git a/charts/loki/loki.yaml b/charts/loki/loki.yaml
index e186fa7..e29b8e9 100644
--- a/charts/loki/loki.yaml
+++ b/charts/loki/loki.yaml
@@ -47,7 +47,7 @@
   ingester:
     chunk_idle_period: 3m
     chunk_block_size: 262144
-    chunk_retain_period: 1m
+    chunk_retain_period: 15m
     max_transfer_retries: 0
     lifecycler:
       ring: