Add external span IDs #757

Job	Run time
cargo check (msrv) (windows-latest)	59s
cargo check (msrv) (macos-latest)	34s
cargo check (msrv) (android) (armv7-linux-androideabi)	24s
cargo clippy (windows-latest)	2m 16s
cargo check (msrv) (ubuntu-latest)	24s
cargo check (msrv) (android) (aarch64-linux-android)	45s
cargo clippy (android) (armv7-linux-androideabi)	1m 2s
cargo clippy (macos-latest)	54s
cargo check (msrv) (android) (x86_64-linux-android)	49s
cargo clippy (ubuntu-latest)	1m 4s
cargo clippy (android) (aarch64-linux-android)	1m 43s
cargo doc (windows-latest)	1m 16s
cargo clippy (android) (x86_64-linux-android)	1m 34s
cargo doc (macos-latest)	23s
formatting	29s
cargo test (windows-latest)	1m 18s
cargo doc (ubuntu-latest)	18s
typos	7s
cargo test (macos-latest)	33s
cargo test (ubuntu-latest)	39s
	17m 31s

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