TopK processor: Add topk processor plugin code

[mirath]

The pluging uses:

• Selectors to pick the metrics that will be filtered
• A group by clause to group the metrics based on metric names and tags
• A list of fields
• An aggregator function to agreggate each field of the grouped metrics
• Once the aggregations for each group are calculated, the plugin orders the groups by each aggregation and returns all the metrics of each group in the top k spots

PR #3691 closed in favor of this one, as I mangled the branch through a rebase

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.

[mirath]

@danielnelson New PR

[danielnelson]

Merged! 🎉

[mapleleafs89]

Is there any prebuilt solution to integrate this topk plugin data into Grafana dashboard or is anyone willing to? I use the telegraf-system-dashboard but it hasn't been updated in roughly a year :(

[glinton]

@mapleleafs89 You might have better luck asking on the community page

[PauloTemenos]

Just found this plugin. Really cool stuff and (possibly) exactly what I need.

However, it is unclear what is this processor actually doing to the data + it is unclear how to actually use that processor...

Can anything be done abou tit? Maybe update the MD file to include an example(s) config and an example of what data might look like before passing through the processor and after passing through the processor?

[glinton]

@PauloTemenos can you open a new issue for the requested documentation additions, please? Thanks

[PauloTemenos]

@glinton
Feature request opened here: #5005

Cheers.