Syslog rate limit design (#1049)

Logging in SONiC is organized with rsyslogd. Each container has its own rsyslogd instance plus a daemon running on a host. The rsyslogd instance which is running on the host is used to collect the messages from within containers and store them at /var/log/syslog path. rsyslog config file are generated from templates:

Container scope:
Multi ASIC: https://github.com/Azure/sonic-buildimage/blob/master/files/image_config/rsyslog/rsyslog-container.conf.j2
Single ASIC: https://github.com/Azure/sonic-buildimage/blob/master/dockers/docker-base/etc/rsyslog.conf

Host scope: 
https://github.com/Azure/sonic-buildimage/blob/master/files/image_config/rsyslog/rsyslog.conf.j2
Currently, each container has hardcoded message rate limiting to avoid receiving flooded log messages:

$SystemLogRateLimitInterval 300
$SystemLogRateLimitBurst 20000
There is no rate limiting configured on host side for now.

The SystemLogRateLimitInterval determines the amount of time that is being measured for rate limiting. The SystemLogRateLimitBurst defines the amount of messages, that have to occur in the time limit of SystemLogRateLimitInterval, to trigger rate limiting. For example, SystemLogRateLimitInterval=300, SystemLogRateLimitBurst=20000, it means that if one daemon generate more than 20000 messages in 300 seconds, rsyslogd will start to drop messages after that(FIFO).

This feature allows user to configure SystemLogRateLimitInterval and SystemLogRateLimitBurst for host, containers.