Flush fix for SimpleLogProcessor #3633

Job	Run time
lint	32m 11s
non-default-examples (opentelemetry-otlp/examples/external-otlp-grpcio-async-std)	10m 1s
test (stable)	18m 17s
non-default-examples (opentelemetry-otlp/examples/basic-otlp)	2m 50s
test (beta)	17m 20s
external-types (opentelemetry)	2m 10s
external-types (opentelemetry-sdk)	1m 54s
coverage	19m 45s
cargo-deny	46s
docs	11m 40s
msrv	4m 52s
	2h 1m 46s

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