Add config descriptions

[cosmo0920]

:desc option fields are shown like this:

$ bundle exec fluentd --show-plugin-config output:grepcounter -p lib/fluent/plugin/
2015-12-09 11:55:27 +0900 [info]: Show config for output:grepcounter
2015-12-09 11:55:27 +0900 [info]: 
input_key: string: <nil> # The target field key to grep out.
Use with regexp or exclude.

regexp: string: <nil> # The filtering regular expression.
exclude: string: <nil> # The excluding regular expression like grep -v.
regexp1: string: <nil>
regexp2: string: <nil>
regexp3: string: <nil>
regexp4: string: <nil>
regexp5: string: <nil>
regexp6: string: <nil>
regexp7: string: <nil>
regexp8: string: <nil>
regexp9: string: <nil>
regexp10: string: <nil>
regexp11: string: <nil>
regexp12: string: <nil>
regexp13: string: <nil>
regexp14: string: <nil>
regexp15: string: <nil>
regexp16: string: <nil>
regexp17: string: <nil>
regexp18: string: <nil>
regexp19: string: <nil>
regexp20: string: <nil>
exclude1: string: <nil>
exclude2: string: <nil>
exclude3: string: <nil>
exclude4: string: <nil>
exclude5: string: <nil>
exclude6: string: <nil>
exclude7: string: <nil>
exclude8: string: <nil>
exclude9: string: <nil>
exclude10: string: <nil>
exclude11: string: <nil>
exclude12: string: <nil>
exclude13: string: <nil>
exclude14: string: <nil>
exclude15: string: <nil>
exclude16: string: <nil>
exclude17: string: <nil>
exclude18: string: <nil>
exclude19: string: <nil>
exclude20: string: <nil>
count_interval: time: <5> # The interval time to count in seconds.
threshold: integer: <nil> # The threshold number to emit.
Emit if count value >= specified value.
Note that this param is not obsolete.

comparator: string: <">=">
less_than: float: <nil> # Emit if count value is less than (<) specified value.
less_equal: float: <nil> # Emit if count value is less than or equal to (<=) specified value.
greater_than: float: <nil> # Emit if count value is greater than (>) specified value.
greater_equal: float: <nil> # This is same with threshold option.
Emit if count value is greater than or equal to (>=) specified value.

output_tag: string: <nil>
tag: string: <nil> # The output tag. Required for aggregate all.
add_tag_prefix: string: <nil> # Add tag prefix for output message.
remove_tag_prefix: string: <nil> # Remove tag prefix for output message.
add_tag_suffix: string: <nil> # Add tag suffix for output message.
remove_tag_suffix: string: <nil> # Remove tag suffix for output message.
remove_tag_slice: string: <nil> # Remove tag parts by slice function.
Note that this option behaves like tag.split('.').slice(min..max).

output_with_joined_delimiter: string: <nil>
delimiter: string: <nil> # Output matched messages after joined with the specified delimiter.
aggregate: string: <"tag"> # Aggregation unit. One of all, in_tag, out_tag can be specified.
replace_invalid_sequence: bool: <false> # Replace invalid byte sequence in UTF-8 with '?' character if true.
store_file: string: <nil> # Store internal count data into a file of the given path on shutdown, and load on statring.

In fluentd 0.12.15 or older, :desc has no effect.

[sonots]

Thanks