chore: remove use of state field in SpanBuilder #263

Job	Run time
cargo check	41s
warnings	43s
cargo fmt	14s
cargo check (feature combinations)	24s
cargo check (+MSRV -Zminimal-versions) (stable)	32s
build tests (wasm32-unknown-unknown)	54s
build tests (wasm32-wasip1)	58s
cargo test (beta on ubuntu-latest)	59s
cargo check (+MSRV -Zminimal-versions) (1.75.0)	28s
cargo test (nightly on ubuntu-latest)	57s
cargo test (stable on windows-latest)	2m 22s
cargo test (stable on macos-latest)	1m 9s
cargo test (stable on ubuntu-latest)	1m 0s
all systems go!	3s
	11m 24s

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