Fix elapsed time calculations

I've found a few places in Rails code base where I think it makes sense
to calculate elapsed time more precisely by using
`Concurrent.monotonic_time`:

- Fix calculation of elapsed time in `ActiveSupport::Cache::MemoryStore#prune`
- Fix calculation of elapsed time in
  `ActiveRecord::ConnectionAdapters::ConnectionPool::Queue#wait_poll`
- Fix calculation of elapsed time in
  `ActiveRecord::ConnectionAdapters::ConnectionPool#attempt_to_checkout_all_existing_connections`
- Fix calculation of elapsed time in `ActiveRecord::ConnectionAdapters::Mysql2Adapter#explain`

See
https://docs.ruby-lang.org/en/2.5.0/Process.html#method-c-clock_gettime
https://blog.dnsimple.com/2018/03/elapsed-time-with-ruby-the-right-way

Related to 7c4542146f

activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb

@@ -185,7 +185,7 @@ module ActiveRecord
           def wait_poll(timeout)
             @num_waiting += 1
 
-            t0 = Time.now
+            t0 = Concurrent.monotonic_time
             elapsed = 0
             loop do
               ActiveSupport::Dependencies.interlock.permit_concurrent_loads do
@@ -194,7 +194,7 @@ module ActiveRecord
 
               return remove if any?
 
-              elapsed = Time.now - t0
+              elapsed = Concurrent.monotonic_time - t0
               if elapsed >= timeout
                 msg = "could not obtain a connection from the pool within %0.3f seconds (waited %0.3f seconds); all pooled connections were in use" %
                   [timeout, elapsed]
@@ -686,13 +686,13 @@ module ActiveRecord
           end
 
           newly_checked_out = []
-          timeout_time      = Time.now + (@checkout_timeout * 2)
+          timeout_time      = Concurrent.monotonic_time + (@checkout_timeout * 2)
 
           @available.with_a_bias_for(Thread.current) do
             loop do
               synchronize do
                 return if collected_conns.size == @connections.size && @now_connecting == 0
-                remaining_timeout = timeout_time - Time.now
+                remaining_timeout = timeout_time - Concurrent.monotonic_time
                 remaining_timeout = 0 if remaining_timeout < 0
                 conn = checkout_for_exclusive_access(remaining_timeout)
                 collected_conns   << conn

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -166,9 +166,9 @@ module ActiveRecord
 
       def explain(arel, binds = [])
         sql     = "EXPLAIN #{to_sql(arel, binds)}"
-        start   = Time.now
+        start   = Concurrent.monotonic_time
         result  = exec_query(sql, "EXPLAIN", binds)
-        elapsed = Time.now - start
+        elapsed = Concurrent.monotonic_time - start
 
         MySQL::ExplainPrettyPrinter.new.pp(result, elapsed)
       end

activesupport/lib/active_support/cache/memory_store.rb

@@ -62,13 +62,13 @@ module ActiveSupport
         return if pruning?
         @pruning = true
         begin
-          start_time = Time.now
+          start_time = Concurrent.monotonic_time
           cleanup
           instrument(:prune, target_size, from: @cache_size) do
             keys = synchronize { @key_access.keys.sort { |a, b| @key_access[a].to_f <=> @key_access[b].to_f } }
             keys.each do |key|
               delete_entry(key, options)
-              return if @cache_size <= target_size || (max_time && Time.now - start_time > max_time)
+              return if @cache_size <= target_size || (max_time && Concurrent.monotonic_time - start_time > max_time)
             end
           end
         ensure

activesupport/lib/active_support/notifications/instrumenter.rb

@@ -137,7 +137,7 @@ module ActiveSupport
 
       private
         def now
-          Process.clock_gettime(Process::CLOCK_MONOTONIC)
+          Concurrent.monotonic_time
         end
 
         if clock_gettime_supported?