check variable buffer is a Buffer instance in MonitorAgentInput

[kazegusuri]

in_monitor_agent plugin collects metrics by checking internal variables by instance_exec. It implicitly asserts that @buffer is a buffer instance when get metrics of buffer_queue_length and buffer_total_queued_size. If a plugin uses the variable name for other purpose, NoMethodError happens.

Example configuration with fluent-plugin-concat which uses @buffer variable as a Hash. On this configuration, doing curl 'http://localhost:24220/api/plugins.json' causes

[error]: #0 NoMethodError in monitoring plugins key="buffer_queue_length" plugin=Fluent::Plugin::ConcatFilter error_class=NoMethodError error="undefined method `queue' for {}:Hash"

<source>
  @type monitor_agent
</source>

<filter **>
   @type concat
    key message
    multiline_start_regexp /foo.bar/
    flush_interval 
    stream_identity_key container_id
    timeout_label @OUT
</filter>

[mururu]

Good catch! Could you add tests for such case?

[kazegusuri]

Thanks. Addeda test, PTAL.

[repeatedly]

Thx!