RUBY-3790 new backpressure-connection-checkin spec test (#3009)

Author: jamis

spec/spec_tests/data/client_backpressure/backpressure-connection-checkin.yml

@@ -0,0 +1,67 @@
+description: tests that connections are returned to the pool on retry attempts for overload errors
+schemaVersion: "1.3"
+runOnRequirements:
+  - minServerVersion: "4.4"
+    topologies:
+      - replicaset
+      - sharded
+      - load-balanced
+createEntities:
+  - client:
+      id: client
+      useMultipleMongoses: false
+      observeEvents:
+        - connectionCheckedOutEvent
+        - connectionCheckedInEvent
+  - client:
+      id: fail_point_client
+      useMultipleMongoses: false
+  - database:
+      id: database
+      client: client
+      databaseName: backpressure-connection-checkin
+  - collection:
+      id: collection
+      database: database
+      collectionName: coll
+tests:
+  - description: overload error retry attempts return connections to the pool
+    operations:
+      - name: failPoint
+        object: testRunner
+        arguments:
+          client: fail_point_client
+          failPoint:
+            configureFailPoint: failCommand
+            mode: alwaysOn
+            data:
+              failCommands:
+                - find
+              errorLabels:
+                - RetryableError
+                - SystemOverloadedError
+              errorCode: 2
+      - name: find
+        object: collection
+        arguments:
+          filter: {}
+        expectError:
+          isError: true
+          isClientError: false
+    expectEvents:
+      - client: client
+        eventType: cmap
+        events:
+          - connectionCheckedOutEvent: {}
+          - connectionCheckedInEvent: {}
+          - connectionCheckedOutEvent: {}
+          - connectionCheckedInEvent: {}
+          - connectionCheckedOutEvent: {}
+          - connectionCheckedInEvent: {}
+          - connectionCheckedOutEvent: {}
+          - connectionCheckedInEvent: {}
+          - connectionCheckedOutEvent: {}
+          - connectionCheckedInEvent: {}
+          - connectionCheckedOutEvent: {}
+          - connectionCheckedInEvent: {}
+