Send client sample rate as a span measurement #14729

Job	Run time
Lint	3m 5s
Lint Rust Default Features	1m 38s
Test (macos-latest)	2m 35s
Test All Features (ubuntu-latest)	3m 43s
Test (windows-latest)	6m 23s
Setup build metadata	1s
Integration Tests	13m 9s
Test Python	59s
Build Relay Binary (relay, x86_64-unknown-linux-gnu)	7m 37s
Build Docker Image (relay)	43s
Sentry-Relay Integration Tests	10m 14s
Publish Relay to DockerHub	0s
Upload build artifacts to gocd	0s
Publish Relay to GCR	0s
self-hosted-end-to-end	13m 9s
	1h 3m 16s

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