[error]: Permission denied - /var/log/httpd/access_log

# vi /etc/init.d/td-agent

DAEMON_ARGS=${DAEMON_ARGS---user td-agent}
TD_AGENT_ARGS="${TD_AGENT_ARGS-/usr/sbin/td-agent --group td-agent --log /var/log/td-agent/td-agent.log}"

改成

DAEMON_ARGS=${DAEMON_ARGS---user root}
TD_AGENT_ARGS="${TD_AGENT_ARGS-/usr/sbin/td-agent --user root --group td-agent --log /var/log/td-agent/td-agent.log}"

References :
職場でよく耳にするfluentdを試してみました - Qiita