Make `with_flattened_span_fields` public #32

Job	Run time
check	45s
check-docs	16s
cargo-hack	27s
cargo-deny check (advisories)	17s
cargo-deny check (bans licenses sources)	18s
Spell Check with Typos	5s
test-msrv	2m 47s
test-versions (stable)	34s
test-versions (beta)	48s
test-versions (nightly)	47s
	7m 4s

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