KAFKA-17841 Use placeholders instead of string concatenation in logs (a…

…pache#17557)

Reviewers: Chia-Ping Tsai <chia7712@gmail.com>

clients/src/main/java/org/apache/kafka/common/config/AbstractConfig.java

@@ -620,7 +620,7 @@ private Map<String, ConfigProvider> instantiateConfigProviders(
                 provider.configure(configProperties);
                 configProviderInstances.put(entry.getKey(), provider);
             } catch (ClassNotFoundException e) {
-                log.error("Could not load config provider class " + entry.getValue(), e);
+                log.error("Could not load config provider class {}", entry.getValue(), e);
                 throw new ConfigException(providerClassProperty(entry.getKey()), entry.getValue(), "Could not load config provider class or one of its dependencies");
             }
         }