Possibly add metrics for newly added Mongo4 connection pool events

[onobc]

Background

While migrating to Mongo client 4.x in #2493 several new connection pool events were noticed (see below). They were marked out of scope for the initial migration in favor of this follow up ticket.

• connectionCheckOutStarted
• connectionCheckOutFailed
• connectionReady
• connectionPoolCleared (earlier thread with question of the value of adding this one)

The MongoDB Connection Monitoring and Pooling specification has details around each of the above events.

Acceptance Criteria

Each of the above connection pool events either has metrics added for it or reasoning around why metrics are not needed.

[codesimplicity]

Looks like this has been implemented some time ago 😉

But I stumbled upon it when looking for an existing issue regarding connectionCheckOutFailed: this currently only decreases the waitQueueSizes gauge but does not record the failure in any way. I should probably raise a separate issue for that but any hint to similar handling would be appreciated.

[shakuzen]

There was recently this issue asking about exception/error tagging with MongoDB client metrics. Does that issue sound like it captures what you're looking for? I think we ideally would like users to tell us what would be useful rather than us introducing new things based on available events without knowing if it helps anyone.

[codesimplicity]

No, that's a different issue as it is about recording details when actually executing commands against the database.

I've now directly created a pull request: #5057
Not sure if that's the correct approach or if you prefer a separate issue 🤷

[codesimplicity]

@shakuzen coming back to my initial comment:

Looks like this has been implemented some time ago 😉

I think this issue can be closed as MongoMetricsConnectionPoolListener covers all of it. Or is there anything missing? (don't want to push, but looked like this issue got lost)