Add service ping database metric operation (average)

Summary

Similar to #215313 (closed), this tracks the effort to add a new Service Ping DatabaseMetric operation for calculating the average of a certain column given an ActiveRecord relation.

Proposal

• Implement batch average method and add as operation to DatabaseMetric (done in !89913 (merged)).
• Update docs to recommend using batch average when applicable (done in !90183 (merged)).
• Create a new instrumentation that uses batch average and test performance (done in !90089 (merged)).