Fix/add toString method for ServerSelector implementations

Author: jyemin

driver-core/src/main/com/mongodb/internal/selector/ReadPreferenceWithFallbackServerSelector.java

@@ -59,4 +59,13 @@ private boolean clusterContainsOlderServers(final ClusterDescription clusterDesc
                 .filter(serverDescription -> serverDescription.getState() == ServerConnectionState.CONNECTED)
                 .anyMatch(serverDescription -> serverDescription.getMaxWireVersion() < minWireVersion);
     }
+
+    @Override
+    public String toString() {
+        return "ReadPreferenceWithFallbackServerSelector{"
+                + "preferredReadPreference=" + preferredReadPreference
+                + ", fallbackReadPreference=" + fallbackReadPreference
+                + ", minWireVersionForPreferred=" + minWireVersion
+                + '}';
+    }
 }

driver-core/src/main/com/mongodb/selector/CompositeServerSelector.java

@@ -102,7 +102,7 @@ public int hashCode() {
 
     @Override
     public String toString() {
-        return "{"
+        return "CompositeServerSelector{"
                + "serverSelectors=" + serverSelectors
                + '}';
     }