feat(spans): extract country code into spans

[DominikB2014]

work towards getsentry/sentry#75230

This PR adds country_code as an indexed span tag. Eventually we would like to create regional metrics, but we have to make a product decision on whether we want country based metrics, or larger region metrics (ex. continent). There are also ~200 countries, which could be high cardinality wise, but at least we have a known limit (lmk if you have thoughts on this!)

[Dav1dde]

We've tried in the past to get rid of the country code as a span attribute, iirc there were discussions with @k-fish and slowly moving it to on demand.

For frontend heavy and globally operating orgs the country code accounts for 80% of the total cardinality observed.

This was the tracking issue.

[DominikB2014]

For frontend heavy and globally operating orgs the country code accounts for 80% of the total cardinality observed.

@Dav1dde thanks for that insight! so it sounds like your suggesting to avoid tagging metrics by country code directly, and it would be preferred to group by some larger region, perhaps continent. But we can still keep country_code in indexed data for sample.

[Dav1dde]

Probably better to tag by geographical region, but obviously that may not be good enough for the product experience.