feat(spans): tag `user.geo.subregion` on mobile spans #14910

Job	Run time
Setup build metadata	1s
Lint	3m 12s
Lint Rust Default Features	1m 32s
Test (macos-latest)	2m 33s
Test All Features (ubuntu-latest)	3m 49s
Integration Tests	7m 53s
Test (windows-latest)	6m 0s
Test Python	1m 37s
Build Relay Binary (relay, x86_64-unknown-linux-gnu)	7m 40s
Build Docker Image	1s
Publish Relay to DockerHub	1s
Sentry-Relay Integration Tests	1s
Upload build artifacts to gocd	1s
Publish Relay to GCR	1s
self-hosted-end-to-end	1s
	34m 23s

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