GODRIVER-3399: PoolClearedError should have TransientTransactionError label appended to it

[zhouselena]

GODRIVER-3399

Summary

Add TransientTransactionError label to `PoolClearedError and created unit test accordingly.

Background & Motivation

(From DRIVERS-1815) In order for withTransaction to retry PoolClearedErrors, they need to have the TransientTransactionError label appended to them by the driver, similar to server selection errors.

[mongodb-drivers-pr-bot]

API Change Report

No changes found!

[matthewdale]

Error "labels" in this context means that the error type must satisfy the LabeledError interface and have a particular label string. See example code that checks if an error has the "TransientTransactionError" label here.

There are a few ways to accomplish that:

1. Wrap the poolClearedError with a driver.Error and add the "TransientTransactionError" label to the labels list.
2. Add a HasErrorLabel(string) bool method to the poolClearedError type and return true if the arg is "TransientTransactionError".

Option 2 is more complex, and it's not clear how we'd determine if a transaction is running when the poolClearedError is created. Option 1 is already used when handling server selection errors while running a transaction (see here), so I recommend that approach.

In the above example, the "TransientTransactionError" label is always added if selectServer returns an error while a transaction is running. However, some errors that server.Connection returns should not have the "TransientTransactionError" label applied. You'll need to check if the returned error is a poolClearedError before adding the "TransientTransactionError" label.

[zhouselena]

@matthewdale I fixed it to wrap the driver.Error and updated the test accordingly! however, I noticed that PoolClearedError is created here as well and passed through a wantConn trydeliver. I'm not super familiar with what exactly wantConn does, but it is a poolClearedError that is being passed through so I was wondering if it also needs to be labelled there?