Define a reason why the fifocache has evicted an item

[dannykopping]

Signed-off-by: Danny Kopping danny.kopping@grafana.com

What this PR does / why we need it:
This PR adds an eviction reason for the fifocache, which will help an operator know if an entry was evicted because the TTL expired vs the cache is full.

Which issue(s) this PR fixes:
N/A

Special notes for your reviewer:
We use a fifocache when running boltdb-shipper for storing ingesters' index caches locally. We do this because we cannot share the cache between ingesters, to prevent missing out chunks that may not exist in other ingesters.
https://github.com/grafana/loki/blob/main/pkg/loki/modules.go#L414-L425

The cache is configured with a fixed size of 200MB, and is currently not configurable; we could consider making this configurable however even in our biggest clusters we're only seeing ~11MB of use currently.

Checklist

• Documentation added
• Tests updated
• Is this an important fix or new feature? Add an entry in the CHANGELOG.md.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/upgrading/_index.md

[grafanabot]

./tools/diff_coverage.sh ../loki-main/test_results.txt test_results.txt ingester,distributor,querier,querier/queryrange,iter,storage,chunkenc,logql,loki

Change in test coverage per package. Green indicates 0 or positive change, red indicates that test coverage for a package fell.

+           ingester	0%
+        distributor	0%
+            querier	0%
+ querier/queryrange	0%
+               iter	0%
+            storage	0%
+           chunkenc	0%
+              logql	0%
+               loki	0%

[kavirajk]

LGTM 👍

Minor suggestions on reason constants.

[sandeepsukhani]

LGTM with a minor non-blocking suggestion

[grafanabot]

./tools/diff_coverage.sh ../loki-main/test_results.txt test_results.txt ingester,distributor,querier,querier/queryrange,iter,storage,chunkenc,logql,loki

Change in test coverage per package. Green indicates 0 or positive change, red indicates that test coverage for a package fell.

+           ingester	0%
+        distributor	0%
+            querier	0%
+ querier/queryrange	0%
+               iter	0%
+            storage	0%
+           chunkenc	0%
+              logql	0%
+               loki	0%

[grafanabot]

./tools/diff_coverage.sh ../loki-main/test_results.txt test_results.txt ingester,distributor,querier,querier/queryrange,iter,storage,chunkenc,logql,loki

Change in test coverage per package. Green indicates 0 or positive change, red indicates that test coverage for a package fell.

+           ingester	0%
+        distributor	0%
+            querier	0%
+ querier/queryrange	0%
+               iter	0%
+            storage	0%
+           chunkenc	0%
+              logql	0%
+               loki	0%