Enable system.log.rotate_... system configuration

[kenhys]

Which issue(s) this PR fixes:

Fixes N/A

What this PR does / why we need it:

In the previous versions, log rotation options were supported as
--log-rotate-age or --log-rotate-size via command line options.

On Windows, as fluentd is launched as a windows service, it is
required to configure again via --reg-winsvc-fluentdopt or edit
fluentdopt registry key for customization.

This approach is not convenient for Windows users, so it may be better
to support more comprehensive solution - customize via configuration
file.

This commit introduces such a configurable parameter in .conf

  <system>
   <log>
     rotate_age 5
     rotate_size 1048576
   </log>
  </system>

Docs Changes:

TODO: https://docs.fluentd.org/deployment/logging#by-config-file should be updated.

Release Note:

N/A

[kenhys]

Is it not appropriate approach to parse in Fluent::Supervisor#initialize?
Above approach is aimed to pass log_rotate_age, and log_rotate_size to LoggerInitializer.new.
But Fluent::SystemConfig returns @log Fluent::Config::Section which fills rotate_age: null, rotate_size: null.

[kenhys]

@log Fluent::Config::Section which fills rotate_age: null, rotate_size: null.

because of it, Fluent::DaemonLogger is not initialized expectedly.

[kenhys]

Hmm, it affects test_config test case.

.F                                                                    
============================================================================================================================================
Failure: test_include(ConfigTest)                                                                                                           
test/test_config.rb:77:in `test_include'                                                                                                    
     74:     assert_equal 'root_config', c['k1']
     75:     assert_equal 'relative_path_include', c['k2']                                                                                  
     76:     assert_equal 'relative_include_in_included_file', c['k3']                                                                      
  => 77:     assert_equal 'absolute_path_include', c['k4']                                                                                  
     78:     assert_equal 'uri_include', c['k5']                                                                                            
     79:     assert_equal 'wildcard_include_1', c['k6']
     80:     assert_equal 'wildcard_include_2', c['k7']
<"absolute_path_include"> expected but was
<nil>

diff:
? "absolute_path_include"
? ni                     

[kenhys]

In lib/fluent/supervisor.rb, the following code doesn't assign <log> section parameters appropriately.
(but format will be assigned 🤔 )

@conf = Fluent::Config.build(config_path: @config_path).elements.find { |e| e.name == 'system' }
@system_config = Fluent::SystemConfig.new(@conf)

<Fluent::Config::Section {"format":"text","time_format":"%Y-%m-%d %H:%M:%S %z","rotate_age":null,"rotate_size":null}>

[kenhys]

Even though master, test_include test_inline may fail.

[kenhys]

test_inline fails because it tries to handle the following .conf.

 k1 root_config
      include dir/config_test_2.conf  #
      include test/tmp/config/config_test_4.conf                                                                                            
      include file://test/tmp/config/config_test_5.conf                                                                                     
      <include config.d/*.conf />
                                                                                                                                            
<source>                                                              
  type http                                                                                                                                 
  port 2222                                                                                                                                 
 </source>

[ashie]

LGTM