Fix base64 encoding issue in stats

[e-dard]

Required for all non-trivial PRs

• Rebased/mergable
• Tests pass
• CHANGELOG.md updated

Fixes #7177.

This PR changes the tags in /debug/vars to use a map[string]string, which ensures they're encoded correctly in the JSON response, as well as saving on some cycles by reducing the amount of sorting and hashing we are doing of various values.

It also fixes some possible bugs:

• opentsdb.Service.Statistics was ignoring any passed in tags;
• udp.Service.Statistics was ignoring any passed in tags;
• subsriber.balancewriter.Statistics was overwriting passed in tags with default values.

@jwilder @sparrc @corylanou

[mention-bot]

@e-dard, thanks for your PR! By analyzing the annotation information on this pull request, we identified @joelegasse and @gunnaraasen to be potential reviewers

[benbjohnson]

👍

[jwilder]

👍

[sparrc]

excellent, thanks!

[sparrc]

can this be cherry-picked into 1.0?

[jwilder]

@sparrc @e-dard Checked an this does not exist on 1.0. It's only in the current nightlies.

[jsternberg]

I think a better way to do this would be just to make a MarshalJSON method on the models.Tags object and change nothing else. If you do something like this, this change is trivial.

func (s *Statistic) MarshalJSON() ([]byte, error) {
    var o struct {
        Name string `json:"name"`
        Tags map[string]string `json:"tags"`
        Values map[string]interface{} `json:"values"`
    }

    o.Name = s.Name
    o.Tags = s.Tags.Map()
    o.Values = s.Values
    return json.Marshal(&o)
}

Then you don't have to make any other changes and you potentially get the performance improvements you're supposed to get from Ben's change to reduce memory allocations.

[e-dard]

@jsternberg that was the initial quick-fix yes. But then looking through the code it was clear there was a lot of extra complexity we could remove.

I don't think we would get any performance improvements with the Tags approach because we always start with a map[string]string in Statistics calls which we then have to convert into models.Tags.