Allow AR instrumentation to be disabled

We want the cache queries to be fast and instrumentation can slow things
down. Additionally you might not want the SolidCache queries showing up
in your logs.

Instrumentation is enabled by default but can we disabled with:

```ruby
  config.cache_store = [ :solid_cache_store, active_record_instrumentation: false ]
```

README.md

@@ -83,6 +83,7 @@ Solid cache supports these options in addition to the universal `ActiveSupport::
 - `max_entries` - the maximum number of entries allowed in the cache (default: `2.weeks.to_i`)
 - `cluster` - a Hash of options for the cache database cluster, e.g { shards: [:database1, :database2, :database3] }
 - `clusters` - and Array of Hashes for separate cache clusters (ignored if `:cluster` is set)
+- `active_record_instrumentation` - whether to instrument the cache's queries (default: `true`)
 
 For more information on cache clusters see [Sharding the cache](#sharding-the-cache)
 ### Cache trimming

app/models/solid_cache/record.rb

@@ -1,8 +1,18 @@
 module SolidCache
   class Record < ActiveRecord::Base
+    NULL_INSTRUMENTER = ActiveSupport::Notifications::Instrumenter.new(ActiveSupport::Notifications::Fanout.new)
+
     self.abstract_class = true
 
     connects_to **SolidCache.connects_to if SolidCache.connects_to
+
+    class << self
+      def disable_instrumentation
+        connection.with_instrumenter(NULL_INSTRUMENTER) do
+          yield
+        end
+      end
+    end
   end
 end
 

lib/solid_cache/cluster/connection_handling.rb

@@ -92,14 +92,20 @@ def reading_shard(normalized_key:)
         with_shard(shard_for_normalized_key(normalized_key)) { yield }
       end
 
+      def active_record_instrumentation?
+        @active_record_instrumentation
+      end
+
       private
         attr_reader :consistent_hash
 
         def with_shard(shard)
-          if shard
-            Record.connected_to(shard: shard) { yield }
-          else
-            yield
+          disable_active_record_instrumentation_if_required do
+            if shard
+              Record.connected_to(shard: shard) { yield }
+            else
+              yield
+            end
           end
         end
 
@@ -131,6 +137,16 @@ def async_if_required
             yield
           end
         end
+
+        def disable_active_record_instrumentation_if_required
+          if active_record_instrumentation?
+            yield
+          else
+            Record.disable_instrumentation do
+              yield
+            end
+          end
+        end
     end
   end
 end