Ensure we don't mutate provided statistics tags

[e-dard]

Required for all non-trivial PRs

• Rebased/mergable
• Tests pass

@jsternberg as we discussed yesterday.

[mention-bot]

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

[jsternberg]

I think this can probably be more easily done by just adding a MergeTags() function that returns a new map. I don't think there's any need to create a new type.

[e-dard]

@jsternberg you mean MergeTags(a, b map[string]string)?

I had a couple of reasons for the StatisticTags type:

• It's impossible to get the arguments the wrong way around and accidentally merge the wrong thing onto the wrong thing;
• It makes it clear what sort of tags we're talking about when we do defaultTags models.StatisticTags on the types, rather than defaultTags map[string]string.

I don't mind changing it to MergeTags(a, b map[string]string) if it's preferred though.

[jsternberg]

It's fine with me as-is 👍

I think the order might be wrong since I think the statTags are supposed to overwrite what's passed in, but I made that mistake originally when I first did this and they don't conflict anyway. We'll fix that another time if it ends up being an issue.