Make SQLLogger implementations final

UPGRADE.md

@@ -12,6 +12,7 @@ The Doctrine\DBAL\Version class is no longer available: please refrain from chec
 ## BC BREAK: the SQLLogger interface has changed
 
 The methods are the same but use scalar type hints, return types, and non-nullable arrays.
+SQLLogger implementations are now final.
 
 Before:
 

lib/Doctrine/DBAL/Logging/DebugStack.php

@@ -7,7 +7,7 @@
 /**
  * Includes executed SQLs in a Debug Stack.
  */
-class DebugStack implements SQLLogger
+final class DebugStack implements SQLLogger
 {
     /**
      * Executed SQL queries.

lib/Doctrine/DBAL/Logging/EchoSQLLogger.php

@@ -8,7 +8,7 @@
 /**
  * A SQL logger that logs to the standard output using echo/var_dump.
  */
-class EchoSQLLogger implements SQLLogger
+final class EchoSQLLogger implements SQLLogger
 {
     /**
      * {@inheritdoc}

lib/Doctrine/DBAL/Logging/LoggerChain.php

@@ -5,7 +5,7 @@
 /**
  * Chains multiple SQLLogger.
  */
-class LoggerChain implements SQLLogger
+final class LoggerChain implements SQLLogger
 {
     /** @var SQLLogger[] */
     private $loggers = [];