feat(spans): Add a metric to count spans per op #9467

Job	Run time
Lint	5m 6s
Lint Rust Default Features	2m 27s
Test (macos-latest)	4m 13s
Test All Features (ubuntu-latest)	3m 29s
Test (windows-latest)	6m 8s
Test Python 3.8	1m 5s
Build Docker Image (amd64, relay)	13m 9s
Integration Tests	15m 5s
Build Docker Image (amd64, relay-pop)	9m 51s
Sentry-Relay Integration Tests	8m 11s
Push GCR Docker Image (relay)	1m 28s
Publish Relay to DockerHub	0s
Push GCR Docker Image (relay-pop)	54s
	1h 11m 6s

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