Add cursor next iteration metric

[etravassos]

This PR introduces a new instrumentation that captures the time to execute a query to fetch records for an Enumerator next batch call.

Note that the existing build_enumerator.iteration instrumentation does not capture the time of the query execution given that the enumerator lazy loads the data and the actual query is only executed in the next batch call.

Motivation

• While investigating job execution times we needed to drill down and separate the time a job takes to retrieve data vs the time it takes to process the data.

Usage

• the new instrumentation can be used as follows

ActiveSupport::Notifications.subscribe("cursor.iteration") do |*args|
  ...
end

[etiennebarrie]

There's at least the ActiveRecordBatchEnumerator that would be interesting to instrument as well. But yes it's a good idea!

[etiennebarrie]

Suggested change

	ActiveSupport::Notifications.instrument("cursor.iteration") do
	ActiveSupport::Notifications.instrument("active_record_cursor.iteration") do

[etravassos]

Hey 👋 @etiennebarrie @Mangara
Pushed the tweak if you could take another look.