in_statsd: add a new input plugin for statsd

[fujimotos]

This is a new plugin that receives metrics via UDP and outputs
aggregated statistics over them. It's designed to be compatible
with the most popular statsd implementation:

 https://github.com/statsd/statsd

For example, if you sends the messages as follows:

$ echo "foo:1|c" > /dev/udp/127.0.0.1/8125
$ echo "foo:10|c" > /dev/udp/127.0.0.1/8125

... then this plugin outputs the following record:

{"type"=>"counter", "bucket"=>"foo", "count"=>11.000000, "rate"=>11.000000}

With this, we can use Fluent Bit as a general solution for
aggregating performance logs.

Signed-off-by: Fujimoto Seiji [email protected]

[fujimotos]

Nah. I think I was overthinking the specification.

Reading #1546:

There are cases where it is preferable to store performance related data in a "logging" backend instead of a traditional metrics backend. Oftentimes this allows for better correlation against related data and more sophisticated querying.

What's actually required is a simpler plugin that can handle statsd
protocol. If aggregation is required, we can do it later on database.