Reduce redis key size by half for Prometheus::ProxyService

- Prometheus::ProxyService is sending over the contents of cache_key
  in with_reactive_cache and the keys are being duplicated by
  ReactiveCaching#full_reactive_cache_key since it calls
  Prometheus::ProxyService::reactive_cache_key

app/services/prometheus/proxy_service.rb

@@ -5,7 +5,7 @@ module Prometheus
     include ReactiveCaching
     include Gitlab::Utils::StrongMemoize
 
-    self.reactive_cache_key = ->(service) { service.cache_key }
+    self.reactive_cache_key = ->(service) { [] }
     self.reactive_cache_lease_timeout = 30.seconds
     self.reactive_cache_refresh_interval = 30.seconds
     self.reactive_cache_lifetime = 1.minute

changelogs/unreleased/smaller-prom-redis-keys.yml

+---
+title: use-smaller-prometheus-redis-keys
+merge_request: 20006
+author:
+type: fixed