Collect Whisper metrics in MailServer

[adambabik]

Problem

We'd like to collect some Whisper metrics to assess Status usage. E.g. we'd like to see a consistent increase in the number of exchanged messages.

Implementation

Add metrics to wnode-status that runs as a MailServer. #523 has some initial work on gathering peers info that can be useful.

Acceptance Criteria

1. The number of exchanged messages,
2. The volume of cached messages.

Notes

Bear in mind that these metrics must be easy to visualize with Grafana.

It's part of status-im/swarms#68

[xackery]

Would prometheus be viable for metrics gathering?

[adambabik]

I plan to write a simple abstraction or use https://github.com/go-kit/kit/tree/master/metrics in order to support multiple collectors.

One huge disadvantage of expvar is no labels support. We have a use case where we'd like to count messages by topic. With expvar it's only possible by using expvar.Map, however, additional logic to prune old topics is required.

Prometheus, on the other hand, has labels support built-in and provides much better integration with tools like Grafana.

Another thing to consider is server vs mobile node. It's ok to use Prometheus in the server node but it does not really make sense to use it in the mobile nodes. Thanks to the mentioned abstraction, we can select a proper tool depending on a use case.

[xackery]

When you speak of abstraction, you mean you just plan to write an interface and wrap prometheus/other to implement it?

[xackery]

Mainly just looking to do some PRs, this is one around my ballpark is why I shot the comment out. If it's claimed I'll lurk for another.

[adambabik]

@xackery great to hear that! This issue is kind of in progress already but I have a different one that is related to metrics: #546. Would you be able to work on that one?

[adambabik]

Done in #558