Adjust redis cache metrics

* Remove `controller` and `action` labels from duration histogram.
* Create a new simple counter for `controller` and `action`.
* Adjust histogram buckets to observe smaller response times.

changelogs/unreleased/bjk-64064_cache_metrics.yml

+---
+title: Adjust redis cache metrics
+merge_request: 30572
+author:
+type: changed

doc/administration/monitoring/prometheus/gitlab_metrics.md

@@ -34,6 +34,9 @@ The following metrics are available:
 | filesystem_writable               | Gauge     | 9.4   | Whether or not the filesystem is writable |
 | filesystem_read_latency_seconds   | Gauge     | 9.4   | Read latency of a specific filesystem |
 | filesystem_readable               | Gauge     | 9.4   | Whether or not the filesystem is readable |
+| gitlab_cache_misses_total         | Counter   | 10.2  | Cache read miss |
+| gitlab_cache_operation_duration_seconds | Histogram   | 10.2  | Cache access time |
+| gitlab_cache_operations_total     | Counter   | 12.2  | Cache operations by controller/action |
 | http_requests_total               | Counter   | 9.4   | Rack request count |
 | http_request_duration_seconds     | Histogram | 9.4   | HTTP response time from rack middleware |
 | pipelines_created_total           | Counter   | 9.4   | Counter of pipelines created |

lib/gitlab/metrics/subscribers/rails_cache.rb

@@ -50,7 +50,8 @@ module Gitlab
         def observe(key, duration)
           return unless current_transaction
 
-          metric_cache_operation_duration_seconds.observe(current_transaction.labels.merge({ operation: key }), duration / 1000.0)
+          metric_cache_operations_total.increment(current_transaction.labels.merge({ operation: key }))
+          metric_cache_operation_duration_seconds.observe({ operation: key }, duration / 1000.0)
           current_transaction.increment(:cache_duration, duration, false)
           current_transaction.increment(:cache_count, 1, false)
           current_transaction.increment("cache_#{key}_duration".to_sym, duration, false)
@@ -63,12 +64,20 @@ module Gitlab
           Transaction.current
         end
 
+        def metric_cache_operations_total
+          @metric_cache_operations_total ||= ::Gitlab::Metrics.counter(
+            :gitlab_cache_operations_total,
+            'Cache operations',
+            Transaction::BASE_LABELS
+          )
+        end
+
         def metric_cache_operation_duration_seconds
           @metric_cache_operation_duration_seconds ||= ::Gitlab::Metrics.histogram(
             :gitlab_cache_operation_duration_seconds,
             'Cache access time',
-            Transaction::BASE_LABELS.merge({ action: nil }),
-            [0.001, 0.01, 0.1, 1, 10]
+            {},
+            [0.00001, 0.0001, 0.001, 0.01, 0.1, 1.0]
           )
         end
 

spec/lib/gitlab/metrics/subscribers/rails_cache_spec.rb

@@ -201,7 +201,15 @@ describe Gitlab::Metrics::Subscribers::RailsCache do
       it 'observes cache metric' do
         expect(subscriber.send(:metric_cache_operation_duration_seconds))
           .to receive(:observe)
-                .with(transaction.labels.merge(operation: :delete), event.duration / 1000.0)
+          .with({ operation: :delete }, event.duration / 1000.0)
+
+        subscriber.observe(:delete, event.duration)
+      end
+
+      it 'increments the operations total' do
+        expect(subscriber.send(:metric_cache_operations_total))
+          .to receive(:increment)
+          .with(transaction.labels.merge(operation: :delete))
 
         subscriber.observe(:delete, event.duration)
       end