Send client sample rate as a span measurement #14749

Job	Run time
Lint	48s
Lint Rust Default Features	1m 15s
Test (macos-latest)	3m 29s
Test (windows-latest)	3m 12s
Test All Features (ubuntu-latest)	2m 47s
Test Python	1m 6s
Setup build metadata	1s
Integration Tests	3m 26s
Build Relay Binary (relay, x86_64-unknown-linux-gnu)	3m 18s
Build Docker Image	1s
Publish Relay to DockerHub	1s
Sentry-Relay Integration Tests	1s
Publish Relay to GCR	1s
Upload build artifacts to gocd	1s
self-hosted-end-to-end	1s
	19m 28s

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