@Override
  protected void log(String configKey, String format, Object... args) {
    // Not using SLF4J's support for parameterized messages (even though it would be more efficient)
    // because it would
    // require the incoming message formats to be SLF4J-specific.
    if (logger.isDebugEnabled()) {
      logger.debug(String.format(methodTag(configKey) + format, args));
    }
  }
Logs to SLF4J at the debug level, if the underlying logger has debug logging enabled. The
underlying logger can be specified at construction-time, defaulting to the logger for {@link
feign.Logger}.
slf4j/src/main/java/feign/slf4j/Slf4jLogger.java