MongoDB command tracing

[yunicot]

Problem Statement

Hello! Currently you don't have tracing for mongodb commands. I found that you already had the same feature request but it closed. I am ready to implement it. I think it is going to be useful not only for me.

Solution Brainstorm

For tracing I propose to use MongoDB\Driver\Monitoring\CommandSubscriber. This interface contains methods for subscribe for commandStarted and commandSucceeded.