Reduce public exposure of noops #4858

Job	Run time
cargo-deny	28s
lint	3m 52s
msrv	2m 20s
external-types (opentelemetry)	1m 5s
non-default-examples (opentelemetry-otlp/examples/basic-otlp)	46s
external-types (opentelemetry-sdk)	1m 15s
test (stable)	3m 21s
external-types (opentelemetry-otlp)	1m 18s
test (beta)	3m 37s
external-types (opentelemetry-zipkin)	1m 10s
coverage	3m 22s
docs	1m 22s
	23m 56s

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