Implement mackerel-plugin-twemproxy

[yoheimuta]

We plan to use this plugin in production. After our test trial in production, we are sure that it's helpful for operators monitoring twemproxy(s).

In my opinion, twemproxy is a somewhat defacto standard proxy for redis and memcached, so I'm posting this PR.

• Review https://github.com/mackerelio/mackerel-agent-plugins/blob/master/CONTRIBUTING.md

[yoheimuta]

ping. (because it passed 13 days)

[stefafafan]

Thanks for the pull-request and sorry for the late reply.
We will take a look at it in a moment so please wait a bit. 🙇

[yoheimuta]

Thanks for the reply 😌

[stefafafan]

I took a look at the code and noticed you decided to skip fragments and server_ejected_at . Is there a reason you made that decision?

[yoheimuta]

@stefafafan

I took a look at the code and noticed you decided to skip fragments and server_ejected_at

Yes, I skip fragments and server_ejected_at.

fragments "# fragments created from a multi-vector request"
https://github.com/twitter/twemproxy#observability

timestamp when server was ejected in usec since epoch
https://github.com/twitter/twemproxy/blob/0f9e1baf06db478065ba43402b975c57a567d00f/src/nc_stats.h#L40

Is there a reason you made that decision?

Yes.

fragments

We don't mind any number of fragments created, because we think fragments is relatively not important for understanding the twemproxy loads in our usage.

server_ejected_at

I couldn't image the graph to point those timestamp data（= タイムスタンプ値をグラフで表現するわかりやすい方法が思い浮かばなかったので、今回の実装から除外しました）.

Summarizing the above, I decided to skip these parameters because other users should just implement to add these when users needed appear.
( I should write this note in pr description 🙇 )

[stefafafan]

@yoheimuta
Thanks for the description.

I decided to skip these parameters because other users should just implement to add these when users needed appear

I see, that makes sense. Perhaps write a note about fragments and server_ejected_at in the README.md might be great if you don't mind.

Other than that, great work!

[yoheimuta]

@stefafafan Thanks for the review.

I wrote a note in the README.md 11beca9.

[stefafafan]

@yoheimuta
Sorry to make you wait, thanks for adding to the readme!

[yoheimuta]

Thanks!