Scrape a Mesos Master process

[wndhydrnt]

This PR adds support for scraping metrics exposed by a Mesos Master process.

It also introduces a change to the CLI flags by specifying exporter.scrape-mode to distinguish between the different modes that mesos_exporter supports. Also the different flags that included the term url have been unified into the flag exporter.url.

For now, I have added only those metrics also exposed by wndhydrnt/mesos-task-exporter (which I'd like to deprecate in favour of this repo) as these are the metrics I use every day.

Together with prometheus/prometheus#1027, these changes make it possible to monitor a Mesos cluster by running an instance of mesos_exporter next to a Mesos process.

[wndhydrnt]

PTAL @antonlindstrom @juliusv @tommyulfsparre

[wndhydrnt]

I forgot one feature in the initial push. The name of the task and the name of the framework that spawned the task are now added as labels to each metric generated from the /monitor/statistics.json endpoint. The names are the same that are visible in the Mesos UI. This simplifies writing expressions that target several tasks of the same app as the names usually do not change while IDs do.

[StephanErb]

What's the status of this pull request? Would love to get to use those new metrics.

[juliusv]

Good question. /cc @wndhydrnt @brian-brazil

[wndhydrnt]

@juliusv thanks for the review. I've updated the code according to your suggestions.

[juliusv]

@wndhydrnt Awesome, thanks! Taking another look!

[juliusv]

Looks :+1: to me otherwise.

[wndhydrnt]

@juliusv addressed your comment regarding _bytes. As the /metrics/snapshot endpoint also exposes megabytes instead of bytes, I've added the functionality to convert the value of a metric by passing a function to the snapshotMetric struct.

Let me know if this is fine with you and I'll squash the commits.

[juliusv]

@wndhydrnt Thanks!

Hmm, I'm wondering if adding two extra constructor functions isn't a bit of over-engineering for that. How about still keeping all of these metric definitions as struct literals instead of function calls and simply have an optional convertFn field that can be nil (when omitted), and it's only applied if it's not nil?

Also, the convertMegabytesToBytes function is so small that you could consider inlining it, though that's up to taste:

  // ...
  convertFn: func(v float64) { return v * 1024 * 1024 },
  // ...

If you keep it, I'd just name it megabytesToBytes. Seems to be more common to omit prefixes like get, compute, convert, etc. from method names if they are already clear without it.

[wndhydrnt]

@juliusv addressed your last comment

[juliusv]

:+1: Awesome, please squash and I'll merge!

[wndhydrnt]

Squashed

[juliusv]

Thanks!