Enable Redis cache key compression

We have a number of keys that are over 1 MB that would be compressed
significantly (over 90%) if this feature were enabled. We had this
disabled by default to ensure backwards compatibility with the previous
Redis store.

We should enable it now because compression significantly reduces the
pressure on the Redis server. Every time a large payload is stored in
Redis, Redis may need to evict many small keys to make room.

This adds a little more CPU overhead on the application workers, which
is why we have an environment variable
(`ENABLE_REDIS_CACHE_COMPRESSION`) to turn if off it it becomes a
problem.

Behind the scenes, Rails implements this by serializing a `compressed`
variable with the `ActiveSupport::Cache::Entry` if the payload size
exceeds the compression threshold (4K by default). When the entry is
deserialized from Redis, if the `compressed` boolean is set, the value
will be decompressed. Thus even in a mixed-deployment where this setting
is not on by default, Rails will be able to decode compressed values
just fine.

Closes https://gitlab.com/gitlab-org/gitlab/issues/198586

changelogs/unreleased/sh-enable-redis-key-compression.yml

+---
+title: Enable Redis cache key compression
+merge_request: 27254
+author:
+type: performance

config/application.rb

@@ -258,7 +258,7 @@ module Gitlab
     # Full list of options:
     # https://api.rubyonrails.org/classes/ActiveSupport/Cache/RedisCacheStore.html#method-c-new
     caching_config_hash = Gitlab::Redis::Cache.params
-    caching_config_hash[:compress] = false
+    caching_config_hash[:compress] = Gitlab::Utils.to_boolean(ENV.fetch('ENABLE_REDIS_CACHE_COMPRESSION', '1'))
     caching_config_hash[:namespace] = Gitlab::Redis::Cache::CACHE_NAMESPACE
     caching_config_hash[:expires_in] = 2.weeks # Cache should not grow forever
     if Gitlab::Runtime.multi_threaded?